OneDrive, Locked Files, and Multiple Users

We switched over to OneDrive a couple of months ago and had an issue editing our bi-monthly magazine. Our designer had it open to make changes but the President was able to open and edit the document as well (she didn't know it was already open). When they saved their respective versions, it created two copies of the file instead of merging all the changes into one.

The designer locked this month's file, thinking that would fix the issue. But today they both managed to open the file and make changes again! The designers changes saved. The President's changes were lost. It didn't create two documents.

Any idea how we can keep this from happening in future? Thank you!!