Top Menu >> Import Data >> ...More >> View Imports
It will list out your imports and will sort them out based off of Supplier and import method. Click View items will filter out all items imported for that Supplier.
Click ...More >> Trash these items
If you suspect you imported data twice for a supplier, you can sort by Supplier by clicking on the Supplier header. One thing to note is that this View will keep a historical log of every import done, so only rows with items will actually have commissions data.