diff --git a/test/api/v3/integration/groups/POST-groups_groupId_join.test.js b/test/api/v3/integration/groups/POST-groups_groupId_join.test.js
index dbb2e00f15..92acf9023f 100644
--- a/test/api/v3/integration/groups/POST-groups_groupId_join.test.js
+++ b/test/api/v3/integration/groups/POST-groups_groupId_join.test.js
@@ -203,6 +203,16 @@ describe('POST /group/:groupId/join', () => {
         await expect(invitedUser.get('/user')).to.eventually.have.nested.property('party._id', party._id);
       });
 
+      it('Issue #12291: accepting a redundant party invite will let the user stay in the party', async () => {
+        await invitedUser.update({
+          'party._id': party._id,
+        });
+        await expect(invitedUser.get('/user')).to.eventually.have.nested.property('party._id', party._id);
+        await invitedUser.post(`/groups/${party._id}/join`);
+
+        await expect(invitedUser.get('/user')).to.eventually.have.nested.property('party._id', party._id);
+      });
+
       it('notifies inviting user that their invitation was accepted', async () => {
         await invitedUser.post(`/groups/${party._id}/join`);
 
diff --git a/website/server/controllers/api-v3/groups.js b/website/server/controllers/api-v3/groups.js
index ed3434c158..6b0d51b2d0 100644
--- a/website/server/controllers/api-v3/groups.js
+++ b/website/server/controllers/api-v3/groups.js
@@ -544,20 +544,23 @@ api.joinGroup = {
       // Check if was invited to party
       const inviterParty = _.find(user.invitations.parties, { id: group._id });
       if (inviterParty) {
-        inviter = inviterParty.inviter;
+        // Check if the user is already a member of the party or not. Only make the user leave the
+        // party if the user is not a member of the party. See #12291 for more details.
+        if (user.party._id !== group._id) {
+          inviter = inviterParty.inviter;
 
-        // If user was in a different party (when partying solo you can be invited to a new party)
-        // make them leave that party before doing anything
-        if (user.party._id) {
-          const userPreviousParty = await Group.getGroup({ user, groupId: user.party._id });
+          // If user was in a different party (when partying solo you can be invited to a new party)
+          // make them leave that party before doing anything
+          if (user.party._id) {
+            const userPreviousParty = await Group.getGroup({ user, groupId: user.party._id });
 
-          if (userPreviousParty.memberCount === 1 && user.party.quest.key) {
-            throw new NotAuthorized(res.t('messageCannotLeaveWhileQuesting'));
+            if (userPreviousParty.memberCount === 1 && user.party.quest.key) {
+              throw new NotAuthorized(res.t('messageCannotLeaveWhileQuesting'));
+            }
+
+            if (userPreviousParty) await userPreviousParty.leave(user);
           }
-
-          if (userPreviousParty) await userPreviousParty.leave(user);
         }
-
         // Clear all invitations of new user
         user.invitations.parties = [];
         user.invitations.party = {};