I'm confused here - please could you explain. Here's what I see would happen:

1. User 1 adds a caption

2. User 2 adds a caption.

3. Both transmit to the server.

4. Server receives the first edit, applies it to the doc, broadcasts the change.

5. Server receives the second edit, sees it is applied to the previous version of the document, rejects the edit.

6. Client that receives the rejection re-bases their edit, attempts to apply it, and it is rejected as it does not conform with the schema.

So the outcome is whichever client hits the server first wins, which is fine. Where's the problem?