I just got a new computer and reinstalling tixati, I'm reloading the torrents. I want to know if a new download will replace a file already downloaded or will I have duplicates?

It depends. When you are loading a torrent and files are already in place (usually: default download location), Tixati will first check files, then start seeding if torrent is 100% complete, or start downloading if not. All paths and file names have to be consistent with the torrent file, otherwise Tixati won't recognize files and will start downloading.

Below is what I found on duplicate torrents in Tixati, not an answer to your question ;)
Tixati won't accept duplicates on the Transfers list, however a duplicate is "a torrent with the same info hash". You can add multiple torrents containing the same files as long as info hash values are different. When you try to load a torrent with the same folder name as another torrent present on the list, but different info hash, Tixati will add a number in brackets at the beginning of the  folder name to separate the files. You can manually remove "(1)" from the name, in order to point to the same files. In that case Tixati presents a confirmation window "Duplicate Transfer Name". So it is possible to load two torrents with different info hashes, pointing to the same files, but to avoid conflicts, all files should be already 100% complete (you can't download the same files from two different torrents to the same location).

As long as the files are in the designated download folder file names unmodified before adding the torrent to tixati it will check the files for completion. no duplicates will be made.