Multiple users able to edit same pdf, then no-one can save

We are having an issue where two users are able to open the same file for editing - without being notified that another user is in the file.  After edits are completed neither user is able to save the file, and they have to save as a different file name, then merge manually.

This is for files stored on a network fileshare.

Is there a setting in the product that we are missing?