Take, for instance, Nurse Deena Ratchet; if I give her an item that award "Placebo" or "Repair," even though she gets these passively from leveling, she can also get it from the item. No idea if the effect stacks, however. This applies to all heroes, and right now I have no save game with which to submit a proper report. When it happens again, I will submit the necessary files.

That is a known issue. The skills given by items are still a work in progress and the devs are looking into it.

I have been wondering about stacking as well.

It makes sense a bit for it to stack - a doctor with proper tools can work better than one without any. Same applies to the repair, duelist, etc. Unfortunately, none of the skills that there are duplicates of so far are particularly easy to test in themselves, and placebo is glitched in general atm, so discovering the stacking effects is proving difficult