when you have a sub report linked to a main report right click on the subreport edit joins. if you remove them and go to the sub report the record selection should be blank. what happens sometimes is if they are removed from the subreport by report right click on the subreport edit joins, they dont always come out of the record selection.
if you delete them from the record selection in the sub the right click on the subreport edit joins and remove them.
then add them in you should only see it once in the sub report record selection.