unique fields - Mailing list pgsql-sql

hi:

I have a fairly large table.

sample_id | chr | cfrom | cto |
-------------------------------------------
1                c2    19       20
2                c2    19       20
3                c2    19       20
1                c5    10       11
3                c5    10       11


(25,000 rows)

I want to find out how many duplications are there for chr, cfrom and cto

a.   c2,19,20 are common to samples 1,2 and 3.

since there will be many instances like that, do I have to loop over
entire rows and find common chr, cfrom and c2 and ouput with
sample_id.
how can I do that.

thanks
adrian