Is it possible for Muramana's active to proc twice on one single-target attack? For example, Poppy's Devastating Blow (Q) or Twisted Fate's Pick a Card (W) add an effect to the following autoattack. On the next autoattack, the spell is used and on-hit effects are applied. Do these abilities, or any other ability, allow Muramana to activate twice?
While certain abilities (like Draven's Spinning Axe, Mordekaiser's Mace of Spades, or Twisted Fate's Pick a Card) modify your auto-attack, their effect triggers as part of the auto-attack, not as an additional effect.
Therefore, Muramana does not trigger once for the ability AND once for the auto-attack, because, for these abilities, the auto-attack is the ability.
Likewise, abilities that cause on-hit effects (such as Ezreal's Mystic Shot or Gangplank's Parrrley) only trigger Muramana once. Abilities that cause multiple on-hit effects (such as Warwick's Infinite Duress) trigger Muramana once per "hit", but do not get an additional trigger from the fact that it is a single-target spell.
Being that those abilities are on hit effects and muramana is an on hit effect. No.
On hit effects stack with eachother but for no reason will they double proc. I don't remember if Muramana is a CD, but it definitly only procs once per hit.
Now if you wanna talk about spells that make your champion attack more than once, then thats a whole different story.