Depending on how you choose to look at it Unknown is either totally derivative or completely original. The Liam Neeson film is a mosaic put together from the plots of other popular films in much the same way a designer patchwork quilt is pieced together from other things.
Aside from rather a lot of well-done car chase scenes the main interest in Unknown is the plot which, as I said, is pieced together from the plots of various spy thrillers and light science fiction films. In Unknown the protagonist is a scientist on his way to a biotech conference who has an automobile accident and, when he wakes up from his four-day coma, discovers that someone else has assumed his identity. This is a well-used setup and the interesting thing about Unknown is that, rather than trying to find a new angle, it takes a number of previous similar films and braids their plots together. After the accident Neeson is confused about who he is, and so are we; is he Matt Damon from the Bourne Identity? or Gary Sinese from The Forgotten? Arnold Schwarzenegger in Total Recall? Frank Sinatra in The Manchurian Candidate? Harrison Ford in Bladerunner? Unknown zigs and zags from one familiar plot to another. We've seen all the endings -- the main suspense is to guess which strand of the braid we will be following when we get to the scunci.