Render issue when the same user is in a group multiple times

### 📝 Describe the bug

The site currently uses only the user ID as a member list ID when viewing a group. However, the same user can be added multiple times.

### 🔄 To Reproduce

1. Add the same user as multiple posts in Gamma
2. Go to /groups/
3. `Encountered two children with the same key`

### 📷 Screenshots

_No response_

### 💡 Additional context

This can be resolved if we use (user_id, post_id) as a list ID instead, as (user_id, group_id, post_id) has a unique constraint in Gamma.