Have Sans and Flowey fought before?

There is the theory that Sans was, or is, a human, and that would explain why he had caused Flowey his fair share of resets. Because Sans is already powerful as a monster, and as a human, he would be unstoppable. Or, maybe Sans sought out Flowey, for papyrus. In Grillby's, Sans says that Papyrus meets a talking flower. Now, we all know he is referring to Flowey. But what if Sans got sick of Papyrus telling him this, and went to find Flowey himself, and found out about his plan to destroy humanity. We don't know which timeline this is in, and it could have been when any of the souls passed through, and he killed Flowey six times, one for every soul. Now, this is just a theory, and is could not be true, but I hope it helps!

Sans is well aware of Flowey for he can bend space and time. As Flowey kills everybody Sans stops him like he tried to do with you in the Genocide run. Sans could not allow that to happen so he had to kill Flowey knowing well enough that Flowey can just reset his save file -- that's why while fighting Sans he says "Even if we do... we'll just end up back here. Right?"

He is saying that even if he kills you, you can just reload your save file. Besides the fact that Sans may not remember some things from different timelines, he can still travel through time. Each timeline is a new Sans but all the Sans' that exist are not aware of each other, however they can all travel through each timeline.

This makes it the same Sans for when you are trying to get Sans' room key. Each time there is a new Sans the reason why he knows how many times you died is because you are in the same timeline. If you change it Sans will still travel to your timeline -- meaning he will know how many times you have died.

To sum it all up Sans knows who Flowey is and he knows what you are doing in Genocide because he saw Flowey do it. As Sans fights you he tries to break your determination like he did with Flowey, but your determination is stronger than Flowey's. Thus, Sans has killed Flowey many times and because of this Flowey gave up after awhile.

I think that sans can travel in the future and sees what happends in the genocide run, pacific run, and the normal run, therefore learning about floweys secrets, EVidencie of this is that he says in the hot dog stand if you say no to buying a hot dog he says yah you better save up for spiders or college and stuff, talking about muppet and temmie