If you're seeing duplicates, it is probably because you both added the same player to your databases manually (not using the method I described). Internally, a player is identified by a unique identifier, and NOT their name. So if you add a player "Corey" to your database and I add a player "Corey" to my database, they are NOT the same player. If I then add Corey to a tournament, save it, send it to you, and you load it and use the "Add All to DB" feature, player "Corey" from my tournament will be added to your database. It will NOT be the same "Corey" that is already in your database, and thus you'll have 2.
Players listed in red on the Database are players who are also in the current tournament. So, if you've done the above and notice there are two Corey's in your database, but one is in red, you should probably delete the one NOT in red. That would leave the Corey that is the same in the both databases.
Use the Info button on the Stats tab to see which tournaments are loaded, complete, and filtered. If all looks good there but you still don't see players, double-click one of the tournaments (on the Info dialog) and look at how many players versus how many database players are in the tournament. If you see fewer database players (especially if you see zero), then the issue is you don't have the correct players in your database and you need to revisit the above steps.