Is it considered bad form to accept an offer from a company but then reject it if you get another one?

I’ve been offered a job with one of the companies I recently interviewed with, but had also been interviewing with another company during that process that I really like. I had a second interview with them right before the offer from the first company and felt really good about it. I don’t want to make a professional mistake by accepting the first offer and then going back on it if this second company offers me their position, but the fit would be better and their benefits are amazing. I’m not used to the process of job offers and protocol, and have anxiety to boot, so any wisdom or experience in this area that you all have would be helpful.

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