Numbering/sequencing sets of same column values

Question

How to do numbering/sequencing for sets of same column values? For example:

Col1 Col2
Andy  1
Chad  1
Bill  1 
Andy  2
Bill  2
Bill  3
Chad  2
Bill  4

Since Andy got 2 values, I want to number it 1 and 2 in Column 2. For Bill, I want to number it 1, 2, 3 and 4 and so on.

Answer 1

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.

Answer 2

That is kind of slow when your list is really long. I've found sorting the column A to Z or small to larger and then using this formula is much faster:

=IF(A2=A1, A1+1,1)

Basically if the value above is the same then add one to the count else start over at 1