While it is true that if you have 5-6 good research papers after that, nobody will care about what you wrote in that paper (assuming it is as bad as you present it), if it is one of your only 3 official publications by the moment of the job search (whether you include it into your CV or not doesn't matter because once you reach the short list, the mathscinet and the grapevine become more important sources of information about you than your own presentation of yourself) and the other two papers are not readily available, you are a toast.
One thing you can do is to check quickly if what you've done is, indeed, well-known in some form. If it is, you are off the hook because knowingly publishing a published result as new is a no-no, so you may just regretfully say that somebody has "undercut" you there and you need to work on another project now. Whether there is any other clean exit depends on many details you haven't provided and many people gave you good advice already, so I'll stop here.
