It is a common technique in animation to reuse body parts like arms, hand and legs when you design a (animated) character. In most programs this is just a matter of copy and paste. Not so in Synfig. When you draw an arm that contains several layers in a group and you have duplicated it, your first instinct would be to just use the group transformation widget or the green origin handle and move the duplicated arm to the other side and then mirror it, but that will mess up your skeleton later, because the origin of the duplicated object will be affected. If you want to move a duplicated part ONLY move it by selecting all its vertex and then drag it. In the example I attached that means selecting all content in the group of the arm and all the content in the group of the hand and ctrl-a all vertex on the canavs. You can also partly drag and mirror the duplicate with the mirror tool, but when you have to adjust the shape later on you have to do it by selecting and dragging its vertex. Again NOT by dragging the green handle or the group transformation widget. I don’t know if that is a bug in the skeleton/bone code or just because you can move things around in Synfig in to many different ways.
In the first sifz I attached, I have a fairly complex arm that I duplicated the right way. Everything works fine and the bones of both arms transform correctly.
In the second sifz I attached, I have the same fairly complex arm that I duplicated the wrong way (I just moved it with the group transformation widget) and as you can see the whole skeleton/bone construction of the duplicated arm is messed up.
Something to remember, because finding this out after your whole character is ready and rigged will surely mess up your evening. Lol.