As an instant, with swift end, it can destroy a creature or planeswalker, but you lose two life and its exiled
In Commander, there are no fatal mistakes
First, they're an excellent counter to Sabrina, one of the best cards in Pokemon Pocket
At the start of your upkeep, you get to exile the top card of your library, hiding it from everyone else

Cards 102 Released Jan 9th 1999 Card List Collection Status You must be logged in to track your collection #1 - Alakazam #2 - Blastoise #3 - Chansey #4 - Charizard #5 - Clefairy #6 - Gyarados #7 - Hitmonchan #8 - Machamp #9 - Magneton #10 - Mewtwo #11 - Nidoking #12 - Ninetales #13 - Poliwrath #14 - Raichu #15 - Venusaur #16 - Zapdos #17 - Beedrill #18 - Dragonair #19 - Dugtrio #20 - Electabuzz #21 - Electrode #22 - Pidgeotto #23 - Arcanine #24 - Charmeleon #25 - Dewgong #26 - Dratini #27 - Farfetch'd #28 - Growlithe #29 - Haunter #30 - Ivysaur #31 - Jynx #32 - Kadabra #33 - Kakuna #34 - Machoke #35 - Magikarp #36 - Magmar #37 - Nidorino #38 - Poliwhirl #39 - Porygon #40 - Raticate #41 - Seel #42 - Wartortle #43 - Abra #44 - Bulbasaur #45 - Caterpie #46 - Charmander #47 - Diglett #48 - Doduo #49 - Drowzee #50 - Gastly #51 - Koffing #52 - Machop #53 - Magnemite #54 - Metapod #55 - Nidoran M #56 - Onix #57 - Pidgey #58 - Pikachu #59 - Poliwag #60 - Ponyta #61 - Rattata #62 - Sandshrew #63 - Squirtle #64 - Starmie #65 - Staryu #66 - Tangela #67 - Voltorb #68 - Vulpix #69 - Weedle #70 - Clefairy Doll #71 - Computer Search #72 - Devolution Spray #73 - Imposter Professor Oak #74 - Item Finder #75 - Lass #76 - Pokemon Breeder #77 - Pokemon Trader #78 - Scoop Up #79 - Super Energy Removal #80 - Defender #81 - Energy Retrieval #82 - Full Heal #83 - Maintenance #84 - PlusPower #85 - Pokemon Center #86 - Pokemon Flute #87 - Pokedex #88 - Professor Oak #89 - Revive #90 - Super Potion #91 - Bill #92 - Energy Removal #93 - Gust of Wind #94 - Potion #95 - Switch #96 - Double Colorless Energy #97 - Fighting Energy #98 - Fire Energy #99 - Grass Energy #100 - Lightning Energy #101 - Psychic Energy #102 - Water Energy Pokellector Mobile Apps

The only real drawback to Flesh Duplicate is its Vanishing 3 ability