Count duplicate rows then delete duplicates

I have two columns of data, and I need to create a third column to count the number of times that same line appears in the document (and then remove all but the first copy of that line).

my data looks as follows (and it is sorted so all duplicate rows appear directly next to each other):

Adam1998 | Jan
Adam1998 | John
Adam1998 | John
Adam1998 | Paul
Adam1998 | Peter
Adam1998 | Peter
Adam1998 | Peter
Adam1999 |John
Adam1999 | Paul


I need this to look as follows:
Adam1998 | Jan | 1
Adam1998 | John | 2
Adam1998 | Paul | 1
Adam1998 | Peter | 3
Adam1999 |John | 1
Adam1999 | Paul | 1

Thank you