Duplicate slurs when parts are created

• With the Untitled Score, go to File > Parts and create a part
• Enter two notes and a slur in the score
• Move the slur you just entered.

Actual result: a second slur is hidden behind the slur
Expected result: Only a single slur is present

Note: cmdAddSlurnow deals with linked elements while undoAddElement also does. That's probably why there are 4 slurs created in total (2 in score, 2 in part). It has been introduced in 49efff0