st: creating a dummy

["Tumur Gunchinmaa" <t.gunchinmaa@stud.man.ac.uk>]

Hi, 
My data is in a long format. I would like to create a dummy which  count the i (unique value variable). Basically, dum=1 if i is specified  more than once.
For example,
id     year   inc
1      80       5000
1      81       5500
1      82       6000
2      81       5000
3      81       6500
3      82       7000

Many thanks,
Gunche
School of Economic Studies
University of Manchester
Oxford Road 
Manchester
M13 9PL
Tel +44-(0)161-275-4624
Fax +44-(0)161-275-4928
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