Mask issue what to do?

Since the effects group is active, I'd suspect there's something like Motion tile or whatever there that simply repeats the content in a mirrored fashion. Simply remove it if undesired.

Mylenium

You'll need to give some more information. What are the effects on the layer? What (if any) transform setting are adjusted? What do the other layers in the comp stack do? Any adjument layers on top?

Can you solo the layer to make sure the flip effect is actually created within the layer?

With some more information we can maybe determine if it's a bug or just an unintended outcome from the comp and effect setup.