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 two clients by first going to one of the two clients you want to merge and clicking on their name.
Then mouse over the Actions button, and you will see a dropdown appear with the option Merge.
selecting the Merge action will bring up a pop up asking you which clients to merge and warn you as once the clients are merged it cannot be undone. You will then have to type the clients name so that you don't accidentally merge the two accounts.