In a previous post I covered the duplicate check that is in the global address book. If contact records do creep into the systems and you have a regular process to look for them. Once you detect them you can merge records together.
Keep in mind this is not a “customer” merge. If you are a related customer record on a party the lower level of the customers accounts receivable records are not going to be merged via this process.
Cheers
Lachlan