Index groups with repeated atoms don't work properly

While investigating #4148 (closed) I accidentally triggered some other buggy behavior.

Index groups like "2 3 1 2" for gmx distance -f traj.xtc -n index.ndx -select "group \"Contacts\"" work fine for analyzing the distances between atom pairs (2,3) and (1,2). However "1 2 2 3" should also work, but does not because the repeated index 2 gets omitted.