Armature distorts mesh on transformation I'm having hard times, figuring out why the rig behaves veirdly (intended to be 2d)

when i'm parenting the plane "wing" to the armature (with vertex groups), on armature rotation the outline of the mesh is not moving and the texture gets distorted, however it moves correctly.

If i export the model, (with animations) the result is the same, just without blurring, a wing is visible at the original position and a second wing is visible on top of that, with correct animation.

things that i checked, that may cause anomaly:


i only have 2 meshes and 2 bones without hidden children (see outliner, at the img)
bones are  not scaled
objects on same layer
all transformation is applied to the meshes
origins are in same place
the 2 vertex groups both only contains the 4 vertices related to their planes
same result if meshes are joined (2vertex groups under same mesh)
looks like weight painting could cause similar effect, but when changing something the result getting worse (all vertices have max weight)
Finally (after 3 days of debugging)

Somehow my wing plane get extruded (i guess)
so there was 2 planes on the exact same position, but the vertex group only moved the one behind the other.