For us experienced users, this can be extremely irritating at times. I typed out a well-thought-out edit but someone else had already submitted another not-as-well-thought-out edit that only covered a couple things I was covering in mine. So I had to refresh the page and type out some other edits again only to find out that another user submitted another edit before me, still only fixing one thing out of some of the other things I was going to fix. Frustrating, yes.
Now the edits that I was going to make seem so minor, that I don't even want to bother making the edits because it would just add another edit to the history and push it that much closer to community wiki, being edit #3 from user #3. Seems ridiculous.
Have the option to bring up the edit that was submitted for that user to review and make a determination from there. The user can then either:
Override the edit that was submitted previously by marking "my edit is already better than that." This should effectively just get rid of the previous edit for being "too minor" since yours already covered it all and exceeded what it covered. Like a "reject and improve" for suggested edits (except it wasn't suggested).
Make further updates to include things that might have been missed by you that were made in the other edit before submitting your finalized edit. Like a "approve and improve" for suggested edits (blah blah).
Discard their edit entirely.
One user taking their time and making one massive substantial edit should be able to override those who continue to make tiny little minor edits one by one. Again, frustrating.
From the comments, here's the original edit I was trying to submit to this post:
Google's "+1" button code causes W3C validation error
I am using this code: <g:plusone annotation="inline"></g:plusone> Which produces these errors: > there is no attribute "annotation" > element "g:plusone" undefined
Obviously I don't remember the exact edit I was trying to submit, but that's close to what I was trying to save all in one edit.