This looks like a great project for me! I have already built a plagiarism checker like that in the past twice so I know what I am doing. From what I can tell, duplichecker is not much different. I understand the importance of a good user UI. Personally I think that the UI of duplichecker is not really that good and I can do better.
I am certain that I can easily build this for you within 7 days, of course once you provide a little more details on the specific wishes that you have regarding its features, premium subscriptions and user logins. I look forward to hearing from you so I can get started fast so it will be done before the weekend. :)