It's hard to tell from what you are saying, but it sounds like you could have a field in the details section that really belongs in the order header section. You should have a group for the order header, then the individual lines of the order in the details. If the entry is based on the whole order, then put the value field in the group header. If the entry is based on the individual line, then the value should be in the details section and place a summary in the group footer.

If you are truly getting multiple detail records of each order line (line 1 appears twice, line 2 appears twice, etc.) then you have a field present that does not belong in the details, but rather in the header. Take a copy of the report and start removing one field at a time until the duplicate lines disappear and you will know which one is the culprit.