When merging letters in mail or creating just one letter from the constituent record one constituent's address pulls up twice. It is just the street number and name that is being duplicated. Checking the constituent record in question they do not have a duplicate line on the record itself. The merge field isn't included twice, or this would be happening with all records and not just this one.
Re-enter the same address as a new address and mark it as preferred
Delete the old address that was duplicating the address line