Hi I was wondering how to duplicate items like masterballs and stuff ,can..

I will not go through the pain to explain the Mew Glitch again, but I will say that if you do the Mew Glitch and encounter Missingno in the process, your 6th item will be duplicated.

Here's the whole fachizzle on using the Mew Glitch and catching Missingno:

1 - Rhydon
2 - Kangaskhan
3 - NidoranM
4 - Clefairy
5 - Spearow
6 - Voltorb
7 - Nidoking
8 - Slowbro
9 - Ivysaur
10 - Exeggutor
11 - Lickitung
12 - Exeggcute
13 - Grimer
14 - Gengar
15 - NidoranF
16 - Nidoqueen
17 - Cubone
18 - Rhyhorn
19 - Lapras
20 - Arcanine
21 - Mew
22 - Gyarados
23 - Shellder
24 - Tentacool
25 - Gastly
26 - Scyther
27 - Staryu
28 - Blastoise
29 - Pinsir
30 - Tangela
33 - Growlithe
34 - Onix
35 - Fearow
36 - Pidgey
37 - Slowpoke
38 - Kadabra
39 - Graveler
40 - Chansey
41 - Machoke
42 - Mr. Mime
43 - Hitmonlee
44 - Hitmonchan
45 - Arbok
46 - Parasect
47 - Psyduck
48 - Drowzee
49 - Golem
51 - Magmar
52 - Mankey
53 - Electabuzz
54 - Magneton
55 - Koffing
58 - Seel
59 - Diglett
60 - Tauros
64 - Farfetch'd
65 - Venonat
66 - Dragonite
70 - Doduo
71 - Poliwag
72 - Jynx
73 - Moltres
74 - Articuno
75 - Zapdos
76 - Ditto
77 - Meowth
78 - Krabby
82 - Vulpix
83 - Ninetales
84 - Pikachu
85 - Raichu
88 - Dratini
89 - Dragonair
90 - Kabuto
91 - Kabutops
92 - Horsea
93 - Seadra
96 - Sandshrew
97 - Sandslash
98 - Omanyte
99 - Omastar
100 - Jigglypuff
101 - Wigglytuff
102 - Eevee
103 - Flareon
104 - Jolteon
105 - Vaporeon
106 - Machop
107 - Zubat
108 - Ekans
109 - Paras
110 - Poliwhirl
111 - Poliwrath
112 - Weedle
113 - Kakuna
114 - Beedrill
116 - Dodrio
117 - Primeape
118 - Dugtrio
119 - Venomoth
120 - Dewgong
123 - Caterpie
124 - Metapod
125 - Butterfree
126 - Machamp
127 - Missingno.
128 - Golduck
129 - Hypno
130 - Golbat
131 - Mewtwo
132 - Snorlax
133 - Magikarp
136 - Muk
138 - Kingler
139 - Cloyster
141 - Electrode
142 - Clefable
143 - Weezing
144 - Persian
145 - Marowak
147 - Haunter
148 - Abra
149 - Alakazam
150 - Pidgeotto
151 - Pidgeot
152 - Starmie
153 - Bulbasaur
154 - Venusaur
155 - Tentacruel
157 - Goldeen
158 - Seaking
163 - Ponyta
164 - Rapidash
165 - Rattata
166 - Raticate
167 - Nidorino
168 - Nidorina
169 - Geodude
170 - Porygon
171 - Aerodactyl
173 - Magnemite
176 - Charmander
177 - Squirtle
178 - Charmeleon
179 - Wartortle
180 - Charizard
185 - Oddish
186 - Gloom
187 - Vileplume
188 - Bellsprout
189 - Weepinbell
190 - Victreebel
409 = Bulbasaur
437 = Charmander
339 = Ninetails
427 = Aerodactyl
265 = Ivysaur

The six ways to get Mew...

1. First, find a Trainer, any Trainer at all, and battle him/her.

After defeating that Trainer, find a Pokemon of yours that has a Special stat of 21 or 277. Now, put it first in your party.

Go where ever you can find a Ditto and battle it. Let it transform into your Pokemon with the 21 Special stat. After it transforms, defeat it. It is safe to switch out to a different Pokemon now to beat it, if desired.

Fly to Lavender Town and walk off the west end of town. Mew should appear...

2. Walk past Nugget Bridge and left at the end of it. See that grassy bridge right there? Well, go in there, but DO NOT VERSUS THE TRAINER. Now, walk around in the grass until you find an Abra. Catch it when you see it, a simple PokeBall without dealing damage should do the trick. SAVE THE GAME. Now, if the Abra was sent to the PC, store 2 of your Pokemon and withdraw Abra. Why deposite 2 Pokemon? Because you need an extra space in your party to staore Mew. With Abra and 4 other Pokemon in your party, Save the game again, if you want. Head up Nugget Bridge, then swing to the right. You will see a Trainer facing the right, he's one of the first Trainers that will enter your screen. Well, you're going to have to battle him. After doing so, walk into Cerulean City again, then walk back out onto Nugget bridge and head a left where the Trainer in the grass is (I'll explain later why you need to do this). What you'll want to do now is position yourself to be directly above the Trainer, but have him still be one step-square below the screen's view. To give you a feeling of what I mean, position yourself on the left side on the grassy bridge, just down far enough so that you can see 4 grass squares in a square formation below you. Got that? Now SAVE. When you're ready, step down one square and press Start really fast. If done correctly, your manu should pop up, and the Trainer should be to the left of the menu. Now go to your Abra and tell it to Teleport to the Cerulean Pokemon Center. Once you arrive, your A, B, and Start button will not function.


Now, you can battle one of two Trainers now.


You can either battle the first Trainer in Misty's Gym (This way is easier) or you can battle the Trainer on the Route to the right of Nugget Bridge with the Slowpoke. Let's go with the Gym Trainer now, though.

When you walk into the Gym, walk in front of the swimming Trainer and defeat him. Now walk out side of the Gym. Save the game, buy PokeBalls, heal your Pokemon, whatever you need. When you're ready, walk up Nugget Bridge. While walking, your menu should suddenly pop up. Simply press B, and... MEW!

3. You can do the above obtainable, but instead of using Teleport, you can use Fly, if you have it.

4. Find a Trainer, any Trainer, and battle him/her. After the battle, fight a wild Pokemon that might have a spacial stat of 21 or 277. If that Pokemon does have a special stat of 21 or 277, then you should be able to find Mew by either going into the route where the Trainer was that you battled before the Pokemon battle or be able to walk off the west end of Lavender and be able to encounter Mew.

5. Through a GameShark.

6. Through encountering Error Pokemon on the east shore of Cinnabar Island or Seafoam Islands. To encounter Mew as an Error Pokemon, try starting the game with your name as "Xavier" (without the quotes), you may be able to find high level Mews, but I cannot promise this.