Once you have caught a Legendary Pokemon in Dynamax Adventures, you cannot catch that Pokemon again. You can rebattle the same Legendary Pokemon as many times as you'd like, but you will be unable to catch it.
Once you have caught all Legendaries and Ultra Beasts, the only viable purpose of Dynamax Adventures is to farm Dynite Ore for competitive items or to shiny hunt non-Legendary Pokemon with 1% shiny chances.