Déjà vu

It promised a lot, but it ruined everything in the end. A film like this shouldn't have had such a totally happy ending, because this way it's all wrong and makes absolutely no sense. I was enjoying it but it left me very disappointed and with no intention of watching it again.
It was very well done, with Doug (Denzel Washington) investigating an explosion on a ship that killed 500 or more people. He is convinced that it is linked to the death of a woman, Claire (Paula Patton). When the Fbi comes in, with a fat Val Kilmer as the agent in charge, I was glad to see that there was nothing of the usual jurisdiction-related fights, they all got on pretty well, so Doug is brought in their own method of investigation. They have a camera that allows them to look in the past: not whenever they want, it is fixed on 4 days before, and Doug suggests to keep it on Claire all the time. He gets attached to her, and thanks to all this they catch the bastard (played by Jim Caviezel), but it's not enough for Doug. What about her?
He had tried earlier to interfere with the past by sending there a note saying where to go to catch the bomber, but the Doug of the past went away before seeing it and his partner saw it, went there and got shot. Doug says that they were able to change a detail, but not the storyline, because only the way he died changed, being shot and dying before , instead of going on the ship and dying in the explosion. This could have been acceptable, but barely.
Doug wants them to send him back in time, to save her. He goes back and saves her, but he's hurt so she takes him to her home and cures him, and I noticed it before the character did: this was good, because although he thought he had saved her, he sees they're creating all the things that Doug had seen when he had gone to her house after her death. This was good, because you can't change the past, for the simple reason that if you did it in the past then it didn't happen then you never saw it happening... I'm not explaining myself well, it'd need more words, but here it was good because it showed that he was creating now all the things that had already been there in the future... ehm... I mean, it was confirming what had happened, the blood traces, his tracks and fingerprints... so he actually didn't change anything... but at this point they go all wrong. He stops and kills the bomber, avoids the explosion, nobody dies, she lives also, and only future Doug dies, which is obvious if you think about it. How were they going to bring him back to the future? It was not possible to have two Dougs. They solved the problem in this way. Future Doug dies so Claire still alive meets the other Doug and the perfect happy ending is served, with the romance perfectly possible... unfortunately it's nonsense! Basically, nobody died and nothing happened.