Same story on v 7.7.6, I suppose this is a bug!
In my test I use Leads and Main group the Report on Status, then I get also this unneeded duplicating!
Testing a bit more here and I think I see a second problem:
When I uses Created By or Assigned To in the Main group of the Report, then I see Record ID’s in the report instead of Created By/Assinged to. And again also the duplication.
Somebody else can confirm this and need to make a case in github?