I created a hand for my robot by starting with the finger, then rigging it, adding constrants, etc.
Then I duplicated the finger along with the bones to create the rest of the fingers.
The question I have is this: How can I join the bones of these fingers into one rig, and then join the complete finger set rig to the rest of the robot (which was created with bone mirroring) without losing all of my hard work? 
If you want to join multiple duplicate mesh/armatures, the most important thing to remember is to name the bones and vertex groups properly before you join them into one . For example, you should first start with naming the bones (and the corresponding vertex groups in the mesh) with a “root” word such as tip_L(or _R), middle_L and base_L, or phlange3_L, phlange2_L and phlange1_L etc. and then duplicate both the mesh and armature .
After duplicating you need to go and edit the names of the bones/vertex groups by adding a “finger ID” such as “index”, “middle”, etc. into the root word so that for each finger you would have something like tip_index_L, middle _index_L, etc. and tip_middle_L,middle_middle_L, etc. for each corresponding finger . Do this for both the bones and vertex groups in the mesh (I know it sound tedious but all it is just copy/pasting … and you would have had to do this individually for each bone if you did it the conventional way anyway) .
After you are done you should select the finger meshes first and join them first and then select the finger armatures and join them . Go into Pose Mode and test to see everything turned out ok .
If everything is good select both the joined armature and mesh and duplicate it and then mirror the new duplicate (I would assume along the X axis) . Now go and switch the “_L” (or “_R”) suffix with “_R” (or “_L”) suffix for each bone/ vertex group (this actually made easy with the armature though, just go into bone edit mode and select all bones -> hit W -> Flip Left/Right Names) . Now you can just join your new hands to the main body .
The renaming of the vertex group part is predicated on the assumption that you have either assigned/weighted them in a particular way . If you used envelopes to get the deformation/assigned weights all you would have to do would be to unparent/reparent the mesh after you change the bone names before you join the fingers so it would be less copy/pasting .
Hope this helps .