First of all, if you have duplicates or created players, depends on the files you import.
Some editors used created players, some of them used duplicates to edit and some of them just the fake players or free agents.
If you have no created players, you are fine in terms of updating, once you applied a live update. because aufter this update, you will have to re-import all the created teams (like Mx, MLS, Bundesliga, J-League, Other Latin, Other European teams and a lot of the national teams etc.). If there would be created players, the players will be added each time you import the teams again, which will make the created players as well to duplicates.
If you are having 0 created players, this can not happen, which is good
If a created player is not assigned to any team but just as some kind of free agent, it is also not an issue.
The best and complete way to check for duplicates is the search funtion in ML (in transfer section) there you can list all players sorted by different categories. Sorting by names will help a lot. You could also sort after countries and check within each country.
A similar method can be used in Edit / transfer mode. select a team with a free slot and go to add a player - a window pops up, where you can also scan for countries etc.
But keep in mind, the database is nearly about 15.000 players
Looking for duplicates will take some time.
And if the names are not edited, you will have to look for stats etc to identify the duplicates