The "Dupes Clause" (shortened from "Duplicates Clause") is a fan modification to the catching rule of the original ruleset.
There are actually many different versions of the Dupes Clause. The most basic is that "If your first encounter in a route is a Pokemon you've caught before, you can ignore it and try to instead catch the second 'mon you encounter.
Some variations include:
- Expanding the Dupes Clause to include evolutionary relatives of Pokemon already caught
- Allowing chances past a second encounter to catch a "new" 'mon.
- If the first encounter is a Dupe, NO catch is allowed period.
Depending on how the game is played, there are two potential reasons for invoking the Dupes Cluase:
- Nuzlocking is meant to force people to use Pokemon they may not have used before. Catching multiple Pokemon of the same species (especially early-game species, as most people are experienced with using early-game species due to how common they are) contradicts this.
- Some people feel catching multiple Pokemon of the same species (family) makes the game too hard by eliminating a player's options. While this goes against the "make the game harder" intent of Nuzlocking, it is still a common reason.
Most Nuzlockers accept the Dupes Clause as being a legitimate modification of the original ruleset, so runs that use the Dupes Clause are still considered true Nuzlocke Challenges (they are allowed into the FCR section of the official Nuzlocke forums).