Short answer: duplicate contacts keep coming back on Android because the same people are still being synced from more than one source. If Google, Samsung, SIM, or imported account data are all active, the address book can keep rebuilding the overlap after every cleanup pass.
This is why contact cleanup sometimes feels broken. The merge itself may be correct, but the underlying source overlap has not been resolved yet. The real fix is understanding which account layers are still writing the same records back into the phone.
The most common reasons duplicates return
Multiple contact sources are still syncing the same people.
Old imports and current sync sources overlap instead of replacing each other.
SIM records and cloud accounts both preserve copies of the same contact.
Merges removed the visible duplicates, but not the source conflict that recreates them.
How to stop the duplicates from reappearing
Identify which contact sources are still active before running another merge pass.
Treat recurring duplicates as a source problem, not only a cleanup problem.
Resolve the overlapping account setup before expecting the address book to stay clean.
When to merge again
Run another merge only after the source overlap is clearer. Otherwise the cleanup may work temporarily but the duplicates can keep returning, which makes the address book feel unreliable again.
If you need the source-merge guide next, open How to Merge Duplicate Contacts From Google, Samsung, and SIM Accounts. If you want the feature route, continue to duplicate contacts.
When duplicate contacts keep coming back, the real problem is usually sync overlap, not failed cleanup.