Hi friends
i need ur advise on this query.I've 2 fields in a table (patient,invoice).
field invoice contains invoice no for a patient.this no can be duplicated for the same patient but not for the different patient.
i mean we can have data like this

patient invoice
pk patient invoice
1 John I001
2 John I001
3 Paul I002

but there should not be like following
pk patient invoice
A1 John I001
A2 John I001
A3 Paul I001 --its wrong
A4 Paul I002

my question is how can i find out such duplicates ?
Thanks