Roll up fields should calculate only distinct records

Hi All,

We have custom two entities A(offer) & B(Investments) and there is 1-->N relationship i.e., one offer can have many investments.

These investments are made by contacts (which is standard entity) and one contact make multiple investments to the same offer (e.g., a contact named Drake can invest 10$ first time in Offer XYZ, he can invest $20 in the same offer again)

we want to show the count of contacts interested in a particular offer, but we are unable to achieve this by using rollup fields (we created roll up field on offer to show all related investments, but this not giving the correct results, as one contact can make multiple investments in the same offer)

How can we show only count of contacts instead of investments? Although Drake has invested two times, he should be counted only once. I need to show the count and a sub grid of investments on the form in offer entity.

All possible solutions are welcome.

Regards,

Persis