If you copy a folder with tasks inside it, the duplicated tasks all will have the same key as it's original. Unlike when you directly copy a task and it will generate a new key for it.
Then when you try to delete the duplicate folder it causes the Dev to give an error where you have an option to close or continue. You can try clicking on continue but the error does not go away.
Could we have it so the tasks inside the folder you want to copy has new keys generated? its just another way of copying tasks.
It's not that the duplicated tasks had the same keys as the original, the copied folder contained the same tasks. I've now changed this so the members will also be copied (and thus will have new keys).