Bugfix: Screens were uploaded/imported twice

Fixes https://trac.openmicroscopy.org.uk/ome/ticket/12250

To test:
Import a plate (doesn't have to be MIAS as the ticket says, could be any other file type, too) while another plate import is still running. Probably test several times, as the bug occured randomly.
Expected result: Each plate should have been imported just once.