As of Jun 27 the an issue with mirror stated in previous comments is still present.
When after duplicating the left (users right from the front viewing angle) all duplicated bones recieve new suffixes after the side prefix (ex. arm.000.L.000).
To alleviate this issue, after naming the bones on the left side, and before you would duplicate the bones follow these steps with bone selected in EDIT MODE;
1. Assign side by using the ‘Armature’ menu and the ‘Autoname Left/Right’
2. Press ‘W’ in the 3D viewport to bring up the specials context menu, and select “Symmetrize” or press ‘Y’ with the context menu still open.
And that’s it.
Sorry for the repeated information, I just personally struggled with this longer than I’d have liked so I’m hoping this will help someone else. I sourced all the information from other comments to this video.
At least until 2.8 is released.
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