Mesh Editing issue

Hello everyone,im working on a leopard model and, im facing a problem with the mesh, after applying mirror to 2 object (the body and the teeth) who had a shape key before, i ve noticed a problem that i have never faced before, every part i i moved or scale produce another semetric mesh of it on the other side of the Y axis as shown on the images below its like there is a hiding mirror modifier on the Y axis

the mesh now looks normal

i selected the face and everything looks fine until now

but at the moment when i transform the mesh ( scal rotat or pos ) i got this

the same happens with the mouth

but the eyes and another foot object wich i didnt added a shape key for them works fine

hope it can be solved