If you assign an NFT to a render that you made and then keep selling it on stockmedia websites that's indeed pointless and not right but technically you remain the owner of the work and thus are allowed to do so.
If someone buys your NFT then he can sell that NFT but the work remains property of its creator.
You actually keep receiving royalty's if someone resells the NFT pointing to your work.
Someone that buys the NFT actually does not buy the ownership to your work, he just buy’s some token that represents a certificate of authenticity to a single copy or edition. He does not get the rights to the work and is not allowed to copy or redistribute it apart from reselling the NFT token.
Thus you making additional copy’s and selling those for few buck’s makes no sense at all.
Its a different scenario if your a 3D stock seller, you can use your own 3D models to setup some scene and render it, then assign an NFT to it and bring it to the cryptoart websites. You can safely keep selling your 3D models but its again different when you assign an NFT to the 3D model.