reimport: add internal dupe test when reimport into empty test

[valentijnscholten]

while working on batch reimport matching I found one more test case we need to add.
adding it here already so we can see it passes now and still passes in the future batch reimport match pr #13889

it covers the case where reimport is creating multiple new findings and some of these new findings are duplicates or eachother.

[mtesauro]

Approved