How to Automate Value entry with matching numbers in a sheet

So here’s my help request. I have a table with a bunch of different item codes in column A. In column C I enter either an X or U next to the code after I adjust it. What I want to happen is that when I enter an X or U, that every other instance of that same code gets an X or U popping up next to it in column C. I’ve tried Vlookup, Index Match, and countifs and I can’t seem to get them to work how I want. Any insight into this, or additional information needed to find a proper formula?