[R] how to simplify a data.frame and add the counts of duplicate rows as a new column

[Simone Gabbriellini]

Hello List,

I would like to simplify a data.frame like this

columnA	columnB
user10		proj12
user10		proj19
user10		proj12

into something like:

columnA	columnB	columnC
user10		proj12		2
user10		proj19		1

I know unique() can simplify the data.frame, but how to count and store the duplicates?

thanks in advance for any help.

best regards,
Simone