Confirm both accounts are theirs
Ask them to prove ownership of both, for example by replying from each address. Never merge on a claim alone.
Decide which account survives
Keep the one with the longer history or the active plan. Tell the customer what will change before you do it.
Check billing first
If both have a plan, decide how to handle the overlap and any refund before merging.
Merge, then check the result
After the merge, confirm tickets and contacts show under the surviving account and send the customer a short confirmation.
Record why it happened
Tag the ticket. Many duplicates come from a sign-up flow problem you can fix.