Access Report Problems

I have a form with a sub form within it for my companies purchase orders that I’m creating at the moment in microsoft access 2003. Everything was working fine, but now when I go to preview report the data I have input into the sub table duplicates on the report. Now I could just hide duplicates, but this problem also means my total at the end of the report is double what it should be.
If anyone has any suggestions please help. Its been a good 6 years since of used access and I’m really stumped.