The most common cause of timesheet duplication is a misunderstanding of the prompt 'OK to flag timesheets as being transferred to account'. This prompt only asks whether to flag the time as posted — it does not ask whether to post it. Clicking Cancel does not prevent the time from being posted to Dimensions.
You can reverse the duplicated timesheets in Dimensions — this will be reflected correctly in reports. If timesheets are duplicating for a different reason, contact support.