This is not a unique ID in the table shown or you would not have three records with the ID of 1.
I suggest that you need a related table that is free of duplicates. That can easily be produced with Import records from your existing data. You can define a field in this new table to auto-enter a serial number to generate your IDs (which will be unique in the new table.)
You can then use two relationships and replace field contents to update your current table. To assign matching IDs to it. Relationship 1 matches by your client field so that replace field contents can copy over the ID from the new table into the original. relationship 2 matches by the newly assigned ID and is what you would use going forward.
I trust in you, and i am positive that the answer you provided will work, however I still lack the skills to make this happen. Could you baby step me through this please. Maybe a diagram showing the relationships. I thought I hit a huge road block for sure with this one, but im knew you would have the answer.
P.S. I have been trying to maybe write a macro in excel to do this for me, but im not having any luck with that. I hope i can get this to work.
I've posted a detailed process in your other thread.