If you've tried to edit the client, but the entity already exists, you will need to use this method to move the product(s).
Moving products from one client to another
Step 1: Navigate to Products
Step 2: Apply filters or select the applicable product records you want to move to another client and then click Move.
Step 3: Find and select the client record where you want to move the filtered/selected product records to and then click Move
Step 4: If the client you moved the products from no longer has any products, you can safely remove the client record. You have thereby effectively merged 2 client records by first moving the product records of the source client to the target client and then subsequently removing the source client.