Suppose someone named Alice makes a free-to-play game called "Tubbytown" and releases it under the creative commons license "Attribution-NonCommercial-ShareAlike," which from the Creative Commons website says
This license lets others remix, adapt, and build upon your work non-commercially, as long as they credit you and license their new creations under the identical terms.
Now suppose the game is a big hit, and everyone is loving it, especially Bob, who loves it so much that he starts to develop expansions and enhancements to Tubbytown, all for free of course and in full compliance with the original license.
But Alice has been developing her own enhancements, and they're pretty similar to Bob's. They both identified similar painpoints in the game, and they both patched them, but Bob released his patch first. Does Bob have any claim over his enhancements, even though he isn't allowed to sell them? Would Alice be prevented from coming up with enhancements to her own game if other people could prove they thought of and released the idea first?
Now, I'd like to take this one step further. Let's imagine that Bob came up with some very cool and unique enhancements to Alice's game that she had not thought of, and he releases them for free because he is restricted from selling them. Could Alice outright claim Bob's "Adapted Material" because he developed it on her original work?
I hope this is clear enough.