See UDN for more info: https://udn.unrealengine.com/s/question/0D5QP00000MfU8l0AF/uanimnotifynotify-is-called-twice-for-the-same-notify-on-server-in-a-multiplayer-game
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-217698 in the post.