Ideally, SafeAssign detects old papers or any other paper that has been passed through its database or the repository of papers and essays submitted through it. SafeAssign flags any content for plagiarism if it matches any other document contained in the database. This means that if you have ever submitted a paper through your university's portal, then you cannot use it again. The same applies to a paper that you download from public databases and/or uploaded by your friends in different universities. They will all be detected since they are old papers.