Two different release groups were added to a series (edit #47300316, edit #47300411). The release groups were then merged (edit #48029087). This left the remaining release group in the series twice. Although a single entity can be in a series multiple times, it is probably seldom the intention when merging. Thus all but one of the relationships should be removed when merging.

On a related but different note, it seems like duplicates don't display right. One of them has no number of the releases, see the attached picture.