diff --git a/src/servers/app-store/catalogue.test.ts b/src/servers/app-store/catalogue.test.ts
index e6dffbf8..71d20ac3 100644
--- a/src/servers/app-store/catalogue.test.ts
+++ b/src/servers/app-store/catalogue.test.ts
@@ -152,3 +152,35 @@ describe('the UI manifest each sidecar carries', () => {
     expect(new Set(roots).size).toBe(roots.length);
   });
 });
+
+describe('the "do you already have one?" question', () => {
+  it('is always offered by anything that can provision', () => {
+    // The rule: a sidecar that would start a container must first let the user point at one they
+    // already run. Offering only 'provisioned' means someone with a working Immich gets a second one,
+    // and only finds out when two libraries disagree.
+    //
+    // Enforced here rather than trusted to the UI, because a new entry added later would otherwise
+    // silently skip the question.
+    for (const entry of CATALOGUE) {
+      if (!entry.modes.includes('provisioned')) continue;
+      expect(entry.modes).toContain('existing');
+    }
+  });
+
+  it('offers "existing" first, so the prompt leads with it', () => {
+    // `modes` is the order the UI presents them in. Leading with "provision one for me" invites a
+    // second instance from someone who already has one.
+    for (const entry of CATALOGUE) {
+      if (!entry.modes.includes('existing')) continue;
+      expect(entry.modes[0]).toBe('existing');
+    }
+  });
+
+  it('asks for a URL wherever an existing instance can be named', () => {
+    // Without one there is nothing to connect to, and "I already have it" cannot be answered.
+    for (const entry of CATALOGUE) {
+      if (!entry.modes.includes('existing')) continue;
+      expect(entry.existingFields?.some((f) => f.key === 'url' && f.required)).toBe(true);
+    }
+  });
+});
diff --git a/src/servers/app-store/effects.ts b/src/servers/app-store/effects.ts
index 25774069..4649d042 100644
--- a/src/servers/app-store/effects.ts
+++ b/src/servers/app-store/effects.ts
@@ -1,5 +1,5 @@
 import { join } from 'node:path';
-import { getOwnerUser, saveServiceConnection } from 'officerdb';
+import { getOwnerUser, saveServiceConnection, getServiceConnection } from 'officerdb';
 import type { InstallEffects, StepContext } from './installer';
 import { preflight } from './preflight';
 import { runSetupScript } from './run-script';
@@ -67,6 +67,26 @@ export function createEffects(): InstallEffects {
       const owner = await getOwnerUser();
       if (!owner) throw new Error('no owner account');
 
+      // Never overwrite a connection the owner already has, unless they said so.
+      //
+      // Found the hard way: installing a sidecar that was already configured replaced its connection
+      // row with the new install's, silently repointing a working service at something else. On a fresh
+      // machine that is harmless; on one with an existing setup it breaks a feature that was fine, and
+      // the only symptom is that it stops working.
+      //
+      // Blocking rather than failing, because there is a sensible answer and the user is the only one
+      // who has it: keep what is there, or replace it deliberately.
+      const current = await getServiceConnection(owner.id, ctx.entry.id as never);
+      if (current && current.url && current.url !== url && ctx.values.replaceConnection !== 'true') {
+        return {
+          status: 'blocked',
+          reason:
+            `${ctx.entry.label} is already connected to ${current.url}. Installing would repoint it to ` +
+            `${url}. Keep the existing connection, or reinstall with "replace connection" to change it.`,
+          completeAt: current.url,
+        };
+      }
+
       // The OWNER's row: it carries the URL and IS the instance. Members get their own rows later, with
       // a null url that inherits this one — see members.ts and the service_connections schema.
       await saveServiceConnection({