It might be better to label them with numbers, otherwise you will run out of letters after "Z". Put this in C1:
=IF(B1="x",SUMPRODUCT((A$1:A1=A1)*(B$1:B1="x")),"")
then copy down.
Hope this helps.
Pete
EDIT: If you really want letters, then you can use this in C1:
=IF(B1="x",CHAR(SUMPRODUCT((A$1:A1=A1)*(B$1:B1="x"))+64),"")
then copy down.
Pete
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