Fpk rules:

General rules
The duel starts once everybody says "go" or "g"
Quitting the game results in a loss
When TvT one person is chosen by each team to say "g" for the whole team
Going to another location means a loss (exception, if you accidentally tp'd in town, then immediately came back)
No disappearing off your opponent's map for more than 10 seconds (=defwhoring?)
No overly defensive play (100% teleport off-screen as soon as you are on the opponent's mini-map)
No avoiding contact with an opponent intentionally with no plans of attacking
Refusal to show gear/inventory results in a loss
No naked killing
No mercenaries
No use of Healing, Rejuvenation, Thawing and Antidote potions during and before a duel
No excessive use of houses, walls
No stupid precasts/prebuffs like Cyclone Armor, Fade, Town BO
No casting of traps/missiles before both players say "g"
Oak from Nature/Wisp/HotO is fine, Fade precast from claws is fine
Enchant from Demon Limb is allowed if it stays in inventory (cube) during the duel
Fade precast from claws is allowed
No other charges from items (only ones listed above)
Clay Golem is always allowed
Replenish Life is capped at 40
No use of Ethereal items (exception - item is Indestructible, has Zod rune or Self-repair/Replenish quantity affix)
Respecing during the duel is not allowed
No more than 10% slow from items
Maximum one 451 poison damage charm
Maximum 20% block allowed (exception: Assassin's Weapon Block)
Maximum block allowed - 20% (exception: Weapon Block)

Resistances
Fire resistance:

85 maximum + 0% absorb
80 maximum + 15% absorb
Cold Resistance:

85 maximum + 0% absorb (really?)
75 maximum + 20% absorb
Lightning resistance:

85 maximum + 0% absorb
80 maximum + 20% absorb
TG is allowed to everyone
Poison Resistance:

90 maximum + 0% PLR
85 maximum + 25% PLR
80 maximum + 75% PLR

Class-specific rules
Assassin (Traps):

Chaos isn't allowed
Only 1 point in Martial Arts is allowed (only DT and DC)
Lightning resist:
DClaw: 80 maximum + 0% absorb
174: 85 maximum + 0% absorb
Barbarian (WW):

Only 1 point in Berserk is allowed
Bash, Concentrate, Frenzy, Double Swing, and Double Throw isn't allowed
No Widowmaker
Necromancer (Bone):

No Poison skills are allowed
No Corpse skills are allowed
Bone Wall is banned
A recast of Bone Armor is allowed
Bone Prison is allowed vs anything that can Teleport (only in 1v1)
Only Amplify Damage allowed of all Curses (usable in TvT)
Fire Golem + Dwarf Star is allowed (not vs fire trappers) (any of you saw those like ever?)
Fire resistance: 85 maximum + 15% sorb or 90 maximum + 0% sorb
Druid (Wind):

Only 1 point in Fire skills is allowed (3 Volcano Pelt is allowed)
Recasting Cyclone, Wolves, Bear, Sage, Ravens, Worm, and Hurricane is allowed

GM Mode:

General rules
The duel starts once everybody said "go" or "g"
Quitting the game results in a loss
When TvT one person is chosen by each team to say "g" for the whole team
Going to another location means a loss (exception - if you accidentally tp'd in town, then immediately came back)
No disappearing off your opponent's map for more than 10 seconds (=defwhoring?)
No overly defensive play (100% teleport off-screen as soon as you are on the opponent's mini-map)
No avoiding contact with an opponent intentionally with no plans of attacking
Refusal to show gear/inventory results in a loss
No naked killing
No mercenaries
No use of Healing, Rejuvenation, Thawing and Antidote potions during and before a duel
No excessive use of houses, walls
No stupid precasts/prebuffs like Cyclone Armor, Fade, Town BO
No casting of traps/missiles before both players say "g"
Oak from Nature/Wisp/HotO is fine, Fade precast from claws is fine, ES from Memory is fine
Enchant from Demon Limb is allowed if it stays in inventory (cube) during the duel
No other charges from items (only ones listed above)
Clay Golem is always allowed (Traps Assassins should use WSM bug to overstack Clay Golems slow)
No Holy Freeze against IAS-based characters and Traps assassin
Maximum Damage Reduced by and Magic Damage Reduced by - 30
Replenish Life is capped at 40
No use of Ethereal items (exception - item is Indestructible, has Zod rune or Self-repair/Replenish quantity)
No Teleport when using Widowmaker
Respecing during the duel is not allowed
No more than 10% slow from items
Maximum one 451 poison damage charm
Ties are null duels
Resistances
If using more than one item granting percent or integer absorb, the maximum resistance to any element is 75

Fire resistance:

85 maximum + 0% absorb
80 maximum + 15% absorb
Cold resistance:

max 400 total cold resistance + 0% absorb
max 320 total cold resistance + 20% absorb
max 250 total cold resistance + 40% cold absorb
If total cold resistance is more than 300, max cold resistance is 75

Lightning resistance:

85 maximum + 0% absorb
80 maximum + 20% absorb
Poison Resistance:

90 maximum + 0% PLR
85 maximum + 25% PLR
80 maximum + 75% PLR
Characters
Amazon:
Slow Missiles is banned
Assassin:
Dual-claw assassins: maximum resistance to any element is 80
W/S assassins: maximum resistance to any element is 85
Dual-claw assassins: only one item granting absorb is allowed
Necromancer:
Bone Wall is banned
Bone prison is allowed vs characters that have Teleport (only in 1v1)
Iron Maiden and Life Tap are banned
Amplify is allowed in TvT, Lower Resist is allowed only in 1v1, others are banned
Fire resistance: 85 maximum + 15% sorb or 90 maximum + 0% sorb
Paladin:
No use of Mediation if Prayer has more than one base point
Holy bolt is banned
Sorceress:
Can't use mana potions if using ES
Can't have %absorb or + max resistance if using ES
Replenish Life is capped at 27 if using ES
%DR is capped at 31 if using ES
Memory runeword can be used as a prebuff
Cold Sorceresses may use Doom runeword
Items
Dracul's Grasp (only for Smiter, but not in SvS)
Exile runeword (only for Smiter, but not in SvS)
Last Wish (really?^)
Doom runeword (Only Cold Sorceress)
Skills
Life Tap
Iron Maiden
Bone Wall
Slow Missiles
LLD rules (30 lvl)
Duels take place on Nightmare difficulty
The duel starts once everybody said "go" or "g"
Quitting the game results in a loss
Going to another location means a loss