Yeah, that's totally fine. I understand your skepticism, as it looks weird when it happens. I saw it once as well and actually generated a second seen to avoid the repetition. Considering that the BIP39 wordlist consists of 2048 words and you will be using 12 or 24, you might notice one word repeated more than once occasionally. I am not good at math to tell you how probable it is, so I can only guess. I am guessing that if you generate 30 to 40 seeds right now, it's likely that you will encounter a word repeated in one of these seeds if not more.