Confluence should warn if multiple people open a page for editing concurrently. JSPWiki has a nice implementation of this - when you open a page for edits you get a huge warning if someone opened the page during last half an hour and hasn't either cancelled or saved. The warning includes the name of the user who has opened the page. Saving (by anyone) removes the lock.

I assumed this was a standard feature and didn't even check if it's there or not but testing for it's existence revealed there's no warnings.