When AnimNotify is set to AnimMontage, these Notify events may be output twice. It can see it if you let another Montage play while the first one is playing. If you stop the first Montage, it will appear to be evaluated as well.
Then, the character play AnimMontage.
Output two shots "Notify Begin"/"Notify End" logs
Output one shot "Notify Begin"/"Notify End" log
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-158197 in the post.