As Maximumbeans mentioned, "duplicating Pokemon" refers to the act of making exact copies of Pokemon you have, even those that can only legitimately be caught once per save file. Whenever you do a trade with another player online, it forces the game to save before it shows the trade animation. This is to ensure that even if you reset the game afterwards, it will still reflect the trade having occurred, so you won't get the Pokemon you traded away back unless you do another trade.
However, if one were to have a backup save file somewhere, they could, say, trade a Pokemon onto another copy/system, replace the first save with the backup, trade the same Pokemon to the other system again, and repeat as much as desired. This would lead to multiple duplicates of the same Pokemon (including even shiny legendaries) with the exact same details, right down to the OT, IDs, and various hidden identifying aspects. Which can then be thrown into Wonder Trades or onto the GTS and such to effectively "break" the trading economy by weakening the value of various rare Pokemon (especially shiny legendary/mythical Pokemon, since such Pokemon have been "locked" to never be shiny in more recent games to discourage save-scumming for shiny legendaries, making events the only way to get them without transferring from Gen 1-7 games). Cloud saves would easily make this kind of process much easier to utilize and more widely abused.
All that said, Nintendo wisely decided not to allow cloud saves for the Pokemon games, did not allow snapshot saves with the Pokemon VC games on 3DS (and are unlikely to change this for any VC-style re-releases on Switch/Switch 2 if they decide to do them), nor have they allowed Pokemon Stadium 1/2 to link with Pokemon Home or any other Pokemon game (so as to maintain cloud save compatibility for the N64 NSO software). So the only way to duplicate Pokemon is with modded consoles and/or 3rd-party tools, which violate Nintendo's EULAs and can get you banned from online play if you're caught using them.