In this particular case, you could store a version in the record. When Mary submits her edit, your backend will note that Mary submitted an edit to revision "5"

In this particular case, you could store a version in the record. When Mary submits her edit, your backend will note that Mary submitted an edit to revision "5" but the current revision is "6", and ask Mary to confirm her changes. You'd just use a database transaction to atomically increment the version number when a new revision is created. I think this is analogous to what Wikipedia does.