I've had this issue twice now. I'll receive a message someone sent once, but I'll get 4 copies of it in a row.
In the thousands of messages I've sent/received, this has happened only twice, so I can't pinpoint what the cause is.
Open to any suggestions and happy to "try to force it to happen" if anyone has any debug ideas.
edit: Using the stock messaging app, not Handcent or anything like that.