If you're used to a sword having a single sharp edge and behave as such while wielding a double-edged sword then the instrument you only intended to harm others

If you're used to a sword having a single sharp edge and behave as such while wielding a double-edged sword then the instrument you only intended to harm others with can unintentionally cause harm to yourself. I mean it's probably good idea to be carefully around swords generally, but the phrase is quite old and was maybe a bit more relatable back when it was coined :)