Unity acting really weird when playing builds

I noticed this really awfully annoying thing with unity, where if you build a standalone, launch it and you also launch your game in the editor itself and play it, it acts REALLY weirdly. Sometimes, both players or cameras are controlled simultaneously by one player, or when I shoot, other player also shoots.

You might say, well fix your scripts, you probably made a lot of mistakes. Well, no, that's not the case, when I play with my friends, everything is ok and runs as expected, no bugs.

Am I the only one that has this stupid problem?