Sharing Task Folder

I have a user that is sharing his Task folder with another user. It seems that yesterday it just started to double all the tasks in the folder, on both computers. What is showing up is the original task and then also the updated task. Shouldn’t justthe updated task be showing (over writing the original)? They both have full rights (mod, write, del …etc) for the folder. Can anyone please tell me what is wrong and how to stop it from doubling everything? Thank You