RESTful POST Once Exactly (POE) variation

How do you control form POSTs from the client (browser) to the server to prevent a double-submit from being processed twice on the server?

Client : POST "/widget/" Expect: 100-continue []
Server : write new transaction id to db
Server response : 100 Continue, POE: {tranid}
Client : POST "/widget/" POE: {tranid} [body]
Server : POE {tranid} exist in db?
Server : delete {tranid}
Server : Create new resource from [body].
Server response : 201 Created, Location: {newuri} [body]
Client : ...

Reference: http://www.mnot.net/drafts/draft-nottingham-http-poe-00.txt