Add key to unique values in the SQl database

Add key to unique values in the SQl database

My SQL data looks like this:
Col1
A
A
A
B
B
C
D
I want to add a key to only unique values. So the end result will look
like this:
Col1 Col2
A 1
A 1
A 1
B 2
B 2
C 3
D 3
How can I do this?