I am trying to offset a opened path creating with pen tool. But when I add oppset in the shape layer and I increase the ammount it gives me a weird shape like in the picture. It automatically creates double stroke that I don't want. Can anybody help me with it?
This is to be expected. An open path has no intrinsic convexity to determine what's inside or outside and thus the normals are applied on both sides. This would not be different elsewhere and you simply cannot use it in this simple way. It's just math. The best way to proceed would simply be to do it in Illustrator where you can apply the offset, expand the appearance and then clean up the paths as needed before importing/ copy & pasting them into AE.