The most common cause of Timesheet duplication is because users misinterpret the "OK to flag timesheets as being transferred to account" message as "do you want to post this time" and so click "Cancel" expecting the time not to have posted into Dimensions.
Please be aware that this message is asking if you wish to flag the Time as being posted, not if you want to post it and therefore even if you click "Cancel" the time will be posted into Dimensions.
It is possible to reverse out the duplicated timesheets in Dimensions which will also be correctly reflected in any reports.
If the above is not related to your issue and you think your Timesheets are duplicating for another reason, please raise a new case online and reference the title of this article.