Why not have the phone repaired?
As for the messages, are those 'extras' all duplicates? Its not clear from what you say, but I suspect you are saying that the fault with the handset has caused you to go over your inclusive texts? Is that right?
If so why not write to O2, complaining about this and asking for the duplicate messages to be credited to your account.
Remember though that when you check your online usage, if you see two (or more) messages only a few seconds apart. It doesn't necessarily mean that the phone is sending duplicate messages. When a message is longer than 160 characters its sent as several 160 character messages. When you are writing the message you can see towards the top of the screen how many messages it will be sent as.