Montage plays two times before OnMontageEndedgets notified

Hi there!
I am making my own crouch system, and only when I play the Montage to go from Standing to Crouched, the montage plays two times before notifying the OnMontageEnded.

The Montage is played the same way other montages are, but it plays one time very quicly, then it plays a second time, very slowly, and only then it notifies the OnMontageEnded.

How do I find out what is causing the problem / debugg it?