How to add the number of events grouped by field

Help i need to add to the end of the line the number of event grouped by id, my input file:

IN FILE:
ID DATE
10123 01-04-2008
10123 06-05-2008
10123 01-09-2009
10123 11-12-2009
10345 01-22-2008
10345 06-15-2008
10127 02-22-2010

OUT FILE:
ID DATE EVENT_NUMBER
10123 01-04-2008 1
10123 06-05-2008 2
10123 01-09-2009 3
10123 11-12-2009 4
10345 01-22-2008 1
10345 06-15-2008 2
10127 02-22-2010 1

Thanks in advance for your help.

Try this:

awk '{print $0 FS a[$1]++ +1}' file

Thank you very much it worked to perfection

awk '{print $0, ++a[$1]}' urfile