diff --git a/.changeset/simplify-extension-attach-to.md b/.changeset/simplify-extension-attach-to.md
new file mode 100644
index 0000000000..2beb00d056
--- /dev/null
+++ b/.changeset/simplify-extension-attach-to.md
@@ -0,0 +1,5 @@
+---
+'@backstage/frontend-plugin-api': minor
+---
+
+**BREAKING**: Simplified the `ExtensionAttachTo` type to only support a single attachment target. The array form for attaching to multiple extension points has been removed. Also removed the deprecated `ExtensionAttachToSpec` type alias.
diff --git a/packages/core-compat-api/src/collectEntityPageContents.test.tsx b/packages/core-compat-api/src/collectEntityPageContents.test.tsx
index fd74a7e89b..5ee1439360 100644
--- a/packages/core-compat-api/src/collectEntityPageContents.test.tsx
+++ b/packages/core-compat-api/src/collectEntityPageContents.test.tsx
@@ -14,7 +14,7 @@
  * limitations under the License.
  */
 
-import { ExtensionAttachToSpec } from '@backstage/frontend-plugin-api';
+import { ExtensionAttachTo } from '@backstage/frontend-plugin-api';
 import { EntityLayout, EntitySwitch, isKind } from '@backstage/plugin-catalog';
 import { JSX } from 'react';
 import { collectEntityPageContents } from './collectEntityPageContents';
@@ -73,7 +73,7 @@ const otherTestContent = (
 function collect(element: JSX.Element) {
   const result = new Array<{
     id: string;
-    attachTo: ExtensionAttachToSpec;
+    attachTo: ExtensionAttachTo;
   }>();
 
   collectEntityPageContents(element, {
diff --git a/packages/frontend-app-api/src/tree/resolveAppTree.test.ts b/packages/frontend-app-api/src/tree/resolveAppTree.test.ts
index bffc156509..80d478646f 100644
--- a/packages/frontend-app-api/src/tree/resolveAppTree.test.ts
+++ b/packages/frontend-app-api/src/tree/resolveAppTree.test.ts
@@ -137,60 +137,6 @@ describe('buildAppTree', () => {
     `);
   });
 
-  it('should create a tree with clones', () => {
-    const tree = resolveAppTree(
-      'a',
-      [
-        { ...baseSpec, id: 'a' },
-        { ...baseSpec, id: 'b', attachTo: { id: 'a', input: 'x' } },
-        {
-          ...baseSpec,
-          id: 'c',
-          attachTo: [
-            { id: 'a', input: 'x' },
-            { id: 'b', input: 'x' },
-          ],
-        },
-        {
-          ...baseSpec,
-          id: 'd',
-          attachTo: [
-            { id: 'b', input: 'x' },
-            { id: 'c', input: 'x' },
-          ],
-        },
-      ],
-      collector,
-    );
-
-    expect(Array.from(tree.nodes.keys())).toEqual(['a', 'b', 'c', 'd']);
-
-    expect(String(tree.root)).toMatchInlineSnapshot(`
-      "<a>
-        x [
-          <b>
-            x [
-              <c>
-                x [
-                  <d />
-                ]
-              </c>
-              <d />
-            ]
-          </b>
-          <c>
-            x [
-              <d />
-            ]
-          </c>
-        ]
-      </a>"
-    `);
-
-    const orphans = Array.from(tree.orphans).map(String);
-    expect(orphans).toMatchInlineSnapshot(`[]`);
-  });
-
   it('should create a tree out of order', () => {
     const tree = resolveAppTree(
       'b',
diff --git a/packages/frontend-app-api/src/tree/resolveAppTree.ts b/packages/frontend-app-api/src/tree/resolveAppTree.ts
index c2054c4bac..bdd9be0fbf 100644
--- a/packages/frontend-app-api/src/tree/resolveAppTree.ts
+++ b/packages/frontend-app-api/src/tree/resolveAppTree.ts
@@ -153,7 +153,6 @@ export function resolveAppTree(
   }
 
   const orphans = new Array<SerializableAppNode>();
-  const clones = new Map<string, Array<SerializableAppNode>>();
 
   // A node with the provided rootNodeId must be found in the tree, and it must not be attached to anything
   let rootNode: AppNode | undefined = undefined;
@@ -164,46 +163,6 @@ export function resolveAppTree(
     // TODO: For now we simply ignore the attachTo spec of the root node, but it'd be cleaner if we could avoid defining it
     if (spec.id === rootNodeId) {
       rootNode = node;
-    } else if (Array.isArray(spec.attachTo)) {
-      // eslint-disable-next-line no-console
-      console.warn(
-        `Extension '${spec.id}' is using multiple attachment points which is deprecated and will be removed in a future release. ` +
-          `Use a Utility API instead to share functionality across multiple locations. ` +
-          `See https://backstage.io/docs/frontend-system/architecture/27-sharing-extensions for migration guidance.`,
-      );
-      let foundFirstParent = false;
-      for (const origAttachTo of spec.attachTo) {
-        let attachTo = origAttachTo;
-
-        if (!isValidAttachmentPoint(attachTo, nodes)) {
-          attachTo =
-            redirectTargetsByKey.get(makeRedirectKey(attachTo)) ?? attachTo;
-        }
-
-        const parent = nodes.get(attachTo.id);
-        if (parent) {
-          const cloneParents = clones.get(attachTo.id) ?? [];
-
-          if (!foundFirstParent) {
-            foundFirstParent = true;
-            node.setParent(parent, attachTo.input);
-          } else {
-            cloneParents.unshift(parent);
-          }
-
-          for (const extraParent of cloneParents) {
-            const clonedNode = new SerializableAppNode(spec);
-            clonedNode.setParent(extraParent, attachTo.input);
-            clones.set(
-              spec.id,
-              clones.get(spec.id)?.concat(clonedNode) ?? [clonedNode],
-            );
-          }
-        }
-      }
-      if (!foundFirstParent) {
-        orphans.push(node);
-      }
     } else {
       let attachTo = spec.attachTo;
       if (!isValidAttachmentPoint(attachTo, nodes)) {
diff --git a/packages/frontend-plugin-api/report.api.md b/packages/frontend-plugin-api/report.api.md
index c83511fc47..83c4035e3c 100644
--- a/packages/frontend-plugin-api/report.api.md
+++ b/packages/frontend-plugin-api/report.api.md
@@ -924,7 +924,7 @@ export interface Extension<TConfig, TConfigInput = TConfig> {
   // (undocumented)
   $$type: '@backstage/Extension';
   // (undocumented)
-  readonly attachTo: ExtensionAttachToSpec;
+  readonly attachTo: ExtensionAttachTo;
   // (undocumented)
   readonly configSchema?: PortableSchema<TConfig, TConfigInput>;
   // (undocumented)
@@ -934,18 +934,10 @@ export interface Extension<TConfig, TConfigInput = TConfig> {
 }
 
 // @public (undocumented)
-export type ExtensionAttachTo =
-  | {
-      id: string;
-      input: string;
-    }
-  | Array<{
-      id: string;
-      input: string;
-    }>;
-
-// @public @deprecated (undocumented)
-export type ExtensionAttachToSpec = ExtensionAttachTo;
+export type ExtensionAttachTo = {
+  id: string;
+  input: string;
+};
 
 // @public (undocumented)
 export interface ExtensionBlueprint<
diff --git a/packages/frontend-plugin-api/src/wiring/createExtension.ts b/packages/frontend-plugin-api/src/wiring/createExtension.ts
index 4d3eeece2c..42611e78ca 100644
--- a/packages/frontend-plugin-api/src/wiring/createExtension.ts
+++ b/packages/frontend-plugin-api/src/wiring/createExtension.ts
@@ -488,40 +488,36 @@ export function createExtension<
       if (options.name) {
         parts.push(`name=${options.name}`);
       }
-      const attachTo = [options.attachTo]
-        .flat()
-        .map(aAny => {
-          const a = aAny as ExtensionDefinitionAttachTo;
-          if (OpaqueExtensionInput.isType(a)) {
-            const { context } = OpaqueExtensionInput.toInternal(a);
-            if (!context) {
-              return '<detached-input>';
-            }
-            let id = '<plugin>';
-            if (context?.kind) {
-              id = `${context?.kind}:${id}`;
-            }
-            if (context?.name) {
-              id = `${id}/${context?.name}`;
-            }
-            return `${id}@${context.input}`;
+      const a = options.attachTo;
+      let attachTo: string;
+      if (OpaqueExtensionInput.isType(a)) {
+        const { context } = OpaqueExtensionInput.toInternal(a);
+        if (!context) {
+          attachTo = '<detached-input>';
+        } else {
+          let id = '<plugin>';
+          if (context?.kind) {
+            id = `${context?.kind}:${id}`;
           }
-          if ('relative' in a && a.relative) {
-            let id = '<plugin>';
-            if (a.relative.kind) {
-              id = `${a.relative.kind}:${id}`;
-            }
-            if (a.relative.name) {
-              id = `${id}/${a.relative.name}`;
-            }
-            return `${id}@${a.input}`;
+          if (context?.name) {
+            id = `${id}/${context?.name}`;
           }
-          if ('id' in a) {
-            return `${a.id}@${a.input}`;
-          }
-          throw new Error('Invalid attachment point specification');
-        })
-        .join('+');
+          attachTo = `${id}@${context.input}`;
+        }
+      } else if ('relative' in a && a.relative) {
+        let id = '<plugin>';
+        if (a.relative.kind) {
+          id = `${a.relative.kind}:${id}`;
+        }
+        if (a.relative.name) {
+          id = `${id}/${a.relative.name}`;
+        }
+        attachTo = `${id}@${a.input}`;
+      } else if ('id' in a) {
+        attachTo = `${a.id}@${a.input}`;
+      } else {
+        throw new Error('Invalid attachment point specification');
+      }
       parts.push(`attachTo=${attachTo}`);
       return `ExtensionDefinition{${parts.join(',')}}`;
     },
diff --git a/packages/frontend-plugin-api/src/wiring/index.ts b/packages/frontend-plugin-api/src/wiring/index.ts
index 6ce1bb4998..3ec4631543 100644
--- a/packages/frontend-plugin-api/src/wiring/index.ts
+++ b/packages/frontend-plugin-api/src/wiring/index.ts
@@ -58,7 +58,6 @@ export {
 export {
   type Extension,
   type ExtensionAttachTo,
-  type ExtensionAttachToSpec,
 } from './resolveExtensionDefinition';
 export {
   type ExtensionDataContainer,
diff --git a/packages/frontend-plugin-api/src/wiring/resolveExtensionDefinition.test.ts b/packages/frontend-plugin-api/src/wiring/resolveExtensionDefinition.test.ts
index e080ab1f64..79275ab03c 100644
--- a/packages/frontend-plugin-api/src/wiring/resolveExtensionDefinition.test.ts
+++ b/packages/frontend-plugin-api/src/wiring/resolveExtensionDefinition.test.ts
@@ -139,43 +139,6 @@ describe('resolveExtensionDefinition', () => {
       id: 'test',
       input: 'children',
     });
-
-    // Test for backward compatibility - runtime still supports multiple attachment points
-    expect(
-      resolveExtensionDefinition(
-        OpaqueExtensionDefinition.toInternal({
-          ...baseDef,
-          attachTo: [
-            baseInpuf.withContext?.({
-              kind: 'k1',
-              input: 'children',
-            }),
-            baseInpuf.withContext?.({
-              kind: 'k2',
-              input: 'children',
-            }),
-            baseInpuf.withContext?.({
-              kind: 'k3',
-              input: 'children',
-            }),
-          ] as any,
-        }),
-        { namespace: 'test' },
-      ).attachTo,
-    ).toEqual([
-      {
-        id: 'k1:test',
-        input: 'children',
-      },
-      {
-        id: 'k2:test',
-        input: 'children',
-      },
-      {
-        id: 'k3:test',
-        input: 'children',
-      },
-    ]);
   });
 });
 
diff --git a/packages/frontend-plugin-api/src/wiring/resolveExtensionDefinition.ts b/packages/frontend-plugin-api/src/wiring/resolveExtensionDefinition.ts
index 605e75bfe8..a314983b25 100644
--- a/packages/frontend-plugin-api/src/wiring/resolveExtensionDefinition.ts
+++ b/packages/frontend-plugin-api/src/wiring/resolveExtensionDefinition.ts
@@ -30,21 +30,13 @@ import {
 } from '@internal/frontend';
 
 /** @public */
-export type ExtensionAttachTo =
-  | { id: string; input: string }
-  | Array<{ id: string; input: string }>;
-
-/**
- * @deprecated Use {@link ExtensionAttachTo} instead.
- * @public
- */
-export type ExtensionAttachToSpec = ExtensionAttachTo;
+export type ExtensionAttachTo = { id: string; input: string };
 
 /** @public */
 export interface Extension<TConfig, TConfigInput = TConfig> {
   $$type: '@backstage/Extension';
   readonly id: string;
-  readonly attachTo: ExtensionAttachToSpec;
+  readonly attachTo: ExtensionAttachTo;
   readonly disabled: boolean;
   readonly configSchema?: PortableSchema<TConfig, TConfigInput>;
 }
@@ -149,45 +141,35 @@ function resolveExtensionId(
 }
 
 function resolveAttachTo(
-  attachTo: ExtensionDefinitionAttachTo | ExtensionDefinitionAttachTo[],
+  attachTo: ExtensionDefinitionAttachTo,
   namespace?: string,
-): ExtensionAttachToSpec {
-  const resolveSpec = (
-    spec: ExtensionDefinitionAttachTo,
-  ): { id: string; input: string } => {
-    if (OpaqueExtensionInput.isType(spec)) {
-      const { context } = OpaqueExtensionInput.toInternal(spec);
-      if (!context) {
-        throw new Error(
-          'Invalid input object without a parent extension used as attachment point',
-        );
-      }
-      return {
-        id: resolveExtensionId(context.kind, namespace, context.name),
-        input: context.input,
-      };
+): ExtensionAttachTo {
+  if (OpaqueExtensionInput.isType(attachTo)) {
+    const { context } = OpaqueExtensionInput.toInternal(attachTo);
+    if (!context) {
+      throw new Error(
+        'Invalid input object without a parent extension used as attachment point',
+      );
     }
-    if ('relative' in spec && spec.relative) {
-      return {
-        id: resolveExtensionId(
-          spec.relative.kind,
-          namespace,
-          spec.relative.name,
-        ),
-        input: spec.input,
-      };
-    }
-    if ('id' in spec) {
-      return { id: spec.id, input: spec.input };
-    }
-    throw new Error('Invalid attachment point specification');
-  };
-
-  if (Array.isArray(attachTo)) {
-    return attachTo.map(resolveSpec);
+    return {
+      id: resolveExtensionId(context.kind, namespace, context.name),
+      input: context.input,
+    };
   }
-
-  return resolveSpec(attachTo);
+  if ('relative' in attachTo && attachTo.relative) {
+    return {
+      id: resolveExtensionId(
+        attachTo.relative.kind,
+        namespace,
+        attachTo.relative.name,
+      ),
+      input: attachTo.input,
+    };
+  }
+  if ('id' in attachTo) {
+    return { id: attachTo.id, input: attachTo.input };
+  }
+  throw new Error('Invalid attachment point specification');
 }
 
 /** @internal */