If you have multiple copies of a client with different details and want to keep all the information in both you will want to merge the two.
You can merge clients by going searching for the primary client and clicking on their name.
Click the Actions button and then click Merge.
Selecting the Merge action will show a pop up asking you which clients to merge into the primary client record.
Keep in mind that this cannot be undone.
You will then have to type the client's name so that you don't accidentally merge the two accounts.