I don't think you'll get flagged by I wouldn't suggest doing it. Even in a series, having the same style/theme to the thumbnail is good but having the same one can confuse people & not watch them in the right order (it has happened!) or not watch them at all since they think they've seen it already purely based on the same thumbnail.
I’m sure if you won’t get flagged, you could if youtube thinks it’s spam. I wouldn’t suggest using the same thumbnail because you might confuse people and they might think that it’s the sam video over again.