One of my favorite homebrews is a VERY strong ale (10%+) made exclusively of two-row with just a hint of fresh raspberries. (Thank you defunct brewery Great Bear, Wasilla, Alaska!) I don’t know if I should enter this as a fruit beer, a barley wine, or experimental. It certainly wouldn’t do well in the barley wine category because of the fruit and it may be too high in alcohol (is there such a thing!) to satisfy the judges in the fruit category.
Is it legal and/or ethical to enter the same beer in multiple categories at the same competition?
It certainly isn’t illegal. I know some competitions will explicitly say it’s against the rules. I think it would fall under the category of “poor sportsmanship” more than anything. At least in principle your beer should get a fair shake no matter which category you enter, so better to minimize the number of entries they have to judge, and let everyone else have the same opportunity.
Oh, and personally it sounds like it would do well in fruit beer to me. “Imperial whatever” frequently wins those oddball categories.