Send notification to user who checked out the Excel. Better use Excel Online
Make user aware that working in Excel Online enables multiple users working/editing on the Excel file. Edit in Excel will lock the file.
Additionally if a user is editing a document in Excel instead of in browser & another user attempts to edit the document then the original user should receive a message that someone else is trying to edit the document. Many people just leave a document up so others can't edit for long periods of time.