I think there's not an easy way of doing it, when I'm in your situation and want to upload something I found, I have to browse the whole thread to see if it's been uploaded already, making a search with the file name would be another option, the problem is that maybe they didn't use that exact name to upload it. In your case with such a big file, that's not an option of course.
I gotta say that users usually post those siterip links found on the internet, not caring about dupes, but I think it's against the rules, unless there's an exception for that kind of links. It happened to me on the contrary, some user uploaded a bunch of files in a single link, and I was uploading individual files that happened to be on that link...my posts were nuked.