Pokémon fans love making things harder for themselves. There's the Nuzlocke Challenge, in which you can only catch the first Pokémon you meet in every area, and fainted Pokémon are considered "dead" and must be released. There's the original Twitch Plays Pokémon, which keeps popping up with more variants, like playing Pokémon Red and Blue simultaneously.
Reddit user Volman99 probably isn't the first person to play through the entire Pokémon series without repeating a single Pokémon, but he certainly is thorough. His rules include playing all the variants, like Red, Blue, and Yellow; being able to have, but not use "HM slaves" - a fan term for a Pokémon used to learn all the useful game progression moves, but generally not a member of the final team - and allowing himself to count Pokémon with multiple evolutionary branches as two separate entities.
"I tried to plan in advance," Volman99 says in a comment, "to avoid screwing myself over... but I made so many constant revisions that it was honestly a little bit of both [planning and winging it]."
The most difficult part of the challenge, he says, was the burnout, which settled in around the time he had to play Sun, Moon, Ultra Sun, and Ultra Moon back-to-back. He also wasn't a big fan of returning to Kanto ten times over the course of the challenge. However, he did contact Pokérus, the super-rare stat-boosting virus, three times, and found one shiny Pokémon, not counting the ones that are part of the game's story. In total, he used 388 unique Pokémon to complete the challenge.
"Will I do it again? Probably not, but I'd like to continue it for as long as it is actually possible as new games come out. It was a really refreshing way to play Pokemon and see how the series evolved over time and I got to use a lot of Pokemon I would probably never touch otherwise.
Some of the standouts were Passimian, Minior, Cacturne, Skuntank, Beedrill and Dustox. (Those last two had to solo Brock and Roxanne respectively)."
Volman99 took screenshots as he went, turning them into an album of his successes and final teams, which you can check out here.