The issue is somewhere deeper in APT, because aptitude also shows breaks twice on this package:
$ aptitude show gdm3 | grep -A 2 Breaks
Breaks: gnome-control-center (< 3.0), gnome-control-center (< 3.0), gnome-orca
(< 2.30.0-2), gnome-orca (< 2.30.0-2), gnome-panel (< 3.0), gnome-panel
(< 3.0), gnome-screensaver (< 2.17.7), gnome-screensaver (< 2.17.7)
Looks like every control file entry with "<<" turns into two entries with "<".
