Orb effect items in Dota 2 do not indicate that they have an Orb effect. Does this mean that I can stack them? If so, can I stack the same item with exactly the same Orb effect?
Orb effects in Dota 2 are called Unique Attack Modifiers - items that have attack modifiers are noted in the individual item descriptions, and they do not stack. That being said, you can still make use of multiple orb effects if you wish (although it is not recommended since they don't stack).
For example, if two attack modifiers are activated by chance, during any given attack one modifier or the other could activate.
The one exception to the above is in the case of ranged heroes using Eye of Skadi and Lifesteal. As it was in Dota 1, Skadi's slow and lifesteal will both stack.
Good explanation UAM working present here http://www.dota2wiki.com/wiki/Unique_Attack_Modifier
Unique Attack Modifiers (formerly known as Orb Effects) are special modifiers that alter a Hero's basic attack to provide an additional effect.
Multiple modifiers do not stack; for each attack one modifier always takes priority.
The order of priority is as follows:
1. Conditional Modifiers - Modifiers that only occur on some attacks and are not controlled by the player, such as Maelstrom (chance-based), Mjollnir (chance-based), and Geminate Attack (cooldown-based). If a Conditional Modifier fails to proc then a Standard or Ability Modifier will be present on the attack (if applicable)
2. Ability Modifiers - Modifiers granted by Hero abilities
UAMs from hero abilities, such as Viper's Poison Attack, will only be overridden by conditional modifiers if they are set to Autocast. If they are Manually cast, they cannot be overridden.
3. Standard Modifiers - Modifiers granted by items, with the oldest one in a Hero's inventory taking priority (priority can be shifted by dropping an item and picking it up again)
There are two known exceptions: Eye of Skadi and Lifesteal can both be active at the same time. (Lifesteal is granted by Morbid Mask, Helm of the Dominator, Mask of Madness, or Satanic.) Eye of Skadi will also stack with Clinkz Searing Arrows.