Two new features are coming to Rocket League later this month: extra item rarities, and the trade-in system.
In the next update, a new set of item drops will be added which will only be obtainable through online matches. The chance of getting "Rare" and "Very Rare" items will increase the longer you play, and when you get one, this hidden timer will reset.
Secondly, a trade-in system is coming to the game which means you'll be able to swap five duplicate items you have for a rarer item. For example, if you trade in five uncommon items, you'll get a rare item out of it.
Bear in mind though, Psyonix is planning to add in player to player item trading soon, so it might be worth hanging on to your more valuable duplicates for now.