Is it mandatory to be cloned after going back in time
According to the present knowledge of human kind, time travel is not possible, it also raises at the moment a row of unexplainable phenomena so it really does not make any sense to criticize a movie that something isn't consistent or logic. What did you expect when you bought the ticket, Hollywood will give you the answer to mankind's greatest question?
Having said that, I don't see why directors and script writers insist that time travelers going to the past must meet their older self. Most scripts would much easily flow if going backwards would be the opposite of jumping ahead. When you travel to the future you obviously disappear. So when you go back, one should simply "arrive back". Same body but with "memories" from the future. Voila, Doug does not need to die to avoid the awkward situation of their love triangle.
Not to mention the likely event of another operation and there will be more and more Dougs.