You can merge contacts (or companies). Here is how you can go about it:
During the merge, all the information related to your client, including addresses, invoices, totals for credit lines and store credit will transfer into the contact.
A merged client can not be reactivated. Please double check that you are selecting the proper contacts for each field. If the wrong contact ends up deleted, you will have to recreate it and start their invoice history over.
Please note that it's impossible to merge an Alice POS user or a company that is part of your network. We have blocked this functionality to prevent accidental deletion.