InfoPath Dev

Here is the problem:

userA and userB has contribute access to a form.

userA opens the form and changes something. Meanwhile userB opens the form and changes something.

userB submits the form and then userA submits the form.

all changes done by userB disappeared.

My question is that any way to implement some kind of mutex?

That is an issue with any file sharing.  Check out can help with that with SharePoint..

For an enterprise application with multiple users, I would recommend a backend database then you can use record locking and have your data protected and available for reporting,etc.