Duplicating a mesh with shape keys and rig distorts mesh I have a minecraft arm mesh set up on the right, with the shape keys working at a value of 1. On the right, I duplicated the mesh first, then the rig over from Edit Mode of the rig, then connected them. I still had the old shape key on the mesh because I don't want to go through all the work of making the mesh perfect again. The second mesh on left, with a shape key ArmB.R at a value of 1, distorts the mesh. They are both equal at 90 degrees. With my knowledge, they both should look like the mesh on the right when both shape key values are at one. Where did I go wrong?

For a closer inspection, I have uploaded the .blend file.
this is one way i found to solve your problem.

FIRST: put the 3d cursor where the copy of the mesh should go. (seting cursor to center then scaling -1 on X then resetting scale also works)

SECOND: duplicate the original mesh and move it to the 3d cursor.

THIRD: replace all the .L s with  .R s (or to the corresponding bone names)

this should let you mirror your shape keyed mesh fairly easily and should also combine (ctrl+j) without deleting or dirupting up your shape keys