Usually when I see duplicate data like this, it's due to joining two tables and in the right most table there are two matches instead of just one. A way to test it would be to temporarily modify the report and just print it as a detailed report and see if the record shows up twice. If so, then you need to check on the data in the second table to make sure you either understand how it works or make sure there isn't garbage in it.

Honestly, since I don't have your report in front of me it is hard to say what the problem could be. But I've had similar problems many times and once I check the backend data I usually find that the "one to many" relationship was actually a "many to many". Using the DISTINCT keyword in the SQL helps clean this up as well.