Problems with antialias when overlapping drawings

There is an issue when we overlap 2 or more drawings with coincident edges. On the final render a thin line appears on the coincident edges.
How do you get rid of it?

We contected suport but none of the solutions they gave us was satisfactory, like using a remove transparecy, or messing with the drawings anti-alias. Both solutions had a bad impact on the filnal image quality.

On the attached image, you can see the thin line on the edge of the arm, over the shirt.

Thanks in advance