You can accomplish this with countif
and a sliding range :
A B
1 val1 =COUNTIF($A$1:A1, A1)
2 valx =COUNTIF($A$1:A2, A2)
and so on.
The formula in column B can be dragged down / autofilled in the column. It anchors to the start of the range and only looks as far down as the value we are numbering; COUNTIF is tallying up the matching values in the preceding set this way.