IF formula that counts up for repeat IDs

I am using a database to keep track of clothing consigners. In my “seller” sheet, my first column is the ID column and I was using LEFT({First Name}, 1) & LEFT({Last Name}, 1) to assign IDs to the sellers.

But now I have an issue where I have 2 sellers with the same initials - so I need the formula to count up as well (IF {ID} IS_SAME, COUNTA, LEFT({First Name}, 1) & LEFT({Last Name}, 1)

So I should be getting this for IDs that continuously counts up - but it’s not working and tells me the text is wrong. HELP!

AB
AB2
CD
CD2
CD3
FE
GH
GH1

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