Following things are disabled in Dota1:
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 probably activated before duel started,or by 3rd party.
As you can see castable things are disabled since it is supposed to be "autoattack" duel without interference and most orb effects and passive skills are disabled since some of them make some enemies very hard to defeat imagine duel with ursa or slark,if they had their passive skills.
Basically "duel" is a bit weaker "doom" on you and your target,here is the list of what doom disables.
For "duel" differences between dota1 and dota2 refer to this discussion,It appears that valve still didn't disable a lot of abilities that can be activated before the duel and grant you bonuses during the duel,like barrage,voodoo restoration...