For me this is definitely a hardware issue and I seem to have a bit closer to a workaround as this is clearly not the solution.
I tried spraying the keyboard with compressed air as many recommends, but to no avail. I got my 15" MBP 2016 with Touch Bar in December 2016, so probably the very first version.
After a while the keyboard started acting up and random buttons stopped working as expected.
Within the last few months the B has been acting as you describe and sometimes it would even type 2 or 3 Bs at the same time.
I tried removing the B and cleaning it as I've been using my Mac outside quite a lot and the dust could've settled inside the keyboard. This didn't help at all.
Then I tried writing without the button on, so I would use the rubber thing under the button directly and I noticed the issue wasn't as regular as before with the button (key) on.
I then tried to add a little piece of paper below the key and heureka ⚠ I almost always get one B, not two, not three, not zero, but 1 B almost every time I type!
As I stated in the beginning: this is merely a workaround as it shouldn't be needed to fix a hardware issue with a piece of paper, but it seems to work.
I'm travelling right now and working at the same time, so I can't afford to have my Mac in for repairs right now and even if I could I probably wouldn't be able to as I move all the time and won't come back to the same place. I'll probably live with this workaround for now, but I need to bring it in for repairs next year, so Apple can replace or fix this issue.
Disclaimer: if you pull off the keys be careful, they might break in the sides. You can use the small pin you use when dismounting your simcard from your phone - it fits almost perfectly. Remove the keys from the top and not from the bottom as the butterfly mechanism are tied in the bottom of the keys.
Hope this helps someone :-)