In my most recent game, sometimes more than one Duplicant will operate a Manual Generator simultaneously. It's likely worth noting that I've only ever seen this occur with the Manual Generator that's been attached to an Automation Grid. While the image I've attached has two Dupes stacked on the generator at present, I have seen up to four Dupes at once!
Also, sometimes one of the Dupes will 'lock' onto the generator and glitch out, stuck in the role and frozen without any Power being produced. Fortunately, I can just use a "Move To" command to get 'em off and working again when this occurs.
Steps to Reproduce
Attach a Manual Generator to an Automation Grid with a Smart Battery and something that draws Power. Wait a while for Dupes to use it, and it should eventually occur.
This issue has not been confirmed by a developer yet.