Pete, the message means what it says, which is that there has already been a submission on that tax-office/reference.

I guess it could be worth checking that (a) the tax office and reference on the employer are entered correctly and don't belong to a different client and (b) you aren't submitting the wrong year. (check it says 2010 at the top of the form)

You can make this happen in the software by clicking the "Re-Submit" button after a completed submission, and sometimes people have been known to do this if HMRC are slow to respond with the succcess message, for some reason.

If none of those ideas give you any joy I suppose it may be worth calling HMRC and finding out exactly *what* has been filed for that tax reference for 2010, just to make sure that it is your submission. Unfortunately the online website doesn't show up to date information on 3rd party filings because there are 2 different HMRC systems, one for 3rd party software, and one for online filing.