fix(signals): add unique member check to withLinkedState (#4932)

Closes #4931

Author: rainerhahnekamp

modules/signals/spec/with-linked-state.spec.ts

@@ -235,4 +235,23 @@ describe('withLinkedState', () => {
       expect(name()).toBe('Mark');
     });
   });
+
+  it('logs a warning if previously defined signal store members have the same name', () => {
+    vi.spyOn(console, 'warn').mockImplementation(() => {});
+
+    const linkedStateFeature = signalStoreFeature(
+      withState({ value: 1 }),
+      withLinkedState(() => ({
+        value: () => 1,
+      }))
+    );
+
+    linkedStateFeature(getInitialInnerStore());
+
+    expect(console.warn).toHaveBeenCalledWith(
+      '@ngrx/signals: SignalStore members cannot be overridden.',
+      'Trying to override:',
+      'value'
+    );
+  });
 });

modules/signals/src/with-linked-state.ts

@@ -1,5 +1,6 @@
 import { linkedSignal, WritableSignal } from '@angular/core';
 import { toDeepSignal } from './deep-signal';
+import { assertUniqueStoreMembers } from './signal-store-assertions';
 import {
   InnerSignalStore,
   SignalsDictionary,
@@ -85,6 +86,7 @@ export function withLinkedState<
       ...store.props,
     });
     const stateKeys = Reflect.ownKeys(linkedState);
+    assertUniqueStoreMembers(store, stateKeys);
     const stateSource = store[STATE_SOURCE] as SignalsDictionary;
     const stateSignals = {} as SignalsDictionary;