I seem to be having an issue with mirroring weights (again). I have X Mirror enabled, painted one side, gone to Data > Vertex Groups > and selected Copy Vertex Group and even Mirror Vertex Group, and even if I rename the copy so it's something.L and something.R, it keeps putting the mirrored version over the top of the one that's already there. What am I doing wrong? How can I fix this?
I don't know if it helps to remind that X-mirror works along the x-axis. Your character would need to face negative Y to take advantage of that. And Object>Apply>Rotation also.