Following things are disabled:
Activable items (all) (you and target)
Abilities (all) (you and target)
Lightning bolt,wind walk,rupture...
Passive abilities from items (some,mostly orb effects and attack modifiers) (you and target)
• Chain lightning, Maelstorm dan Mjolnir
• True Strike, MKB
• Maim, Sange, Heaven Halberd, dan Sange and Yasha
• Feedback, Diffusal Blade
• True Sight, Gem
• Immolation, Radiance
• Damage Block, Stout shield, PMS, Vanguard.
• Cold attack (RANGE), Eye of skadi
• Critical Strike, Buriza
• Evasion, The Butterfly, Talisman, dan Heaven Halberd.
• Bash, Cranium basher, Abysal Blade
• Lifesteal, Mask of Death (and all other lifesteal items, except Unholy Rage - actived
• Splash Damage, Battlefury
Passive abilities (some,mostly orb effects and attack modifiers) (you and target)
• Frostmourne, Abaddon
• Geminate Attack, Weaver
• Headshot, Sniper
• Frost Attack, Dragon Knight Elder Form level 3
• Incapacitating Bite, Broodmother
• Mana Break, Magina
• Geostrike, Meepo
• Caustic Final, Sand King
• Poison sting, Venomancer
• Phase Shift Auto cast, Puck ( not blocked )
• Fury Swipes, Ursa
• Split shot, Medusa
• Last Word, Silencer
• Permanent invisibility, Rikimaru
• Kraken Shell, Tide
• Bash, Void (and all other bash skills, except Greater bash - Barathrum)
• Blur, Mortred (and all other evasion skill, except Psedu-evasion Backtrack -
• Great Cleave, Sven (and all other Cleave skill, including Tidebringer - Kunkka)
• Coup degrace, Mortred (and all other critical skills, no exceptions!)
If you ever see the animation of bashing or mana burn,or activated items they were activated duel started.
As you can see castable things are disabled since it is supossed to be "autoattack" duel without interferance and most orb effects and passive skills are disabled since some of them make targeted enemies impossible to defeat imagine duel with ursa or slark,if they had their passive skills it would be clear suicide.
Basically the duel casts "doom" on you and your target here is even larger list.