explain

Pretty certain that straight girls trade clothes all the time. I believe it's a bonding thing, like combing eachother's hair.

-They borrowed each others' clothes
-The both bought the same outfits and wore them opposite
-Daphne was cold so Velma switched with her (subcategory of reason 1)
-they both bought the same outfits and were trying to match but accidentally wore the wrong ones. They both went to change at the same time and the problem continued. (Subcategory of reason 2).
-They're secretly strippers and only have one outfit between them (subcategory of reason 1)

I still subscribe to the belief that they dated while Fred and Shaggy dated

I hadn't seen the clothes thing but the way they look at each other don't lie.

Velma argues "the boys like you more because you dress like a slut, if I wore that outfit and you wore my sweater, Freddy would be drooling on my thigh in under 3 minutes."
Daphne replies "Challenge accepted. Loser has to dress like the winner for a month."

Fred: And then Velma was just pourin' out of this thing...
Shaggy: like but where was Scoobs?
Fred: .......
Scooby: .......Reenut wutter! Woih!