If you have duplicate customers, we can suggest two options:
You can delete the oldest version, and keep the newest.
This archives the customer meaning you can restore them later if needed.
It is recommended that you make a note of this action on the remaining customer.
The second option is more time consuming, and that would be to re-create any jobs and records under the one that has the most history, and then archive the duplicate.
In this way, you will have all the history under one contact, and not need to refer to an archived one.