The knowledge of what happened during the Holocaust will keep people from letting it happen again but if those in denial succeed in spreading their alternative facts to enough people then it's very much possible that something similar could happen again. In Sweden we've had extreme right groups trying to sabotage speeches by Holocaust survivors or spreading flyers with the purpose of calling out their "lies".
Prior to another Holocaust there would be clear signs that something like that would be about to happen but if no one believes them then who's going to put a stop to it?
And I think we all know there are forces in the western world who are more or less open about promoting genocide and happily would get rid of jews and other unwanted people if they gained the political power to do so.
