if i have a lifeline in play and my thicket elemental dies can i pay the kicker again when it comes back. Also, does lifeline work on multiple creatures.
You can ONLY pay kicker when you cast it from your hand or "as though it was in your hand". That would mean from your hand or stuff like Grinning Totem, Word of Command, or Yawgmoth's Will. It does not mean Bribery, Temporal Aperture, or any recursion stuff that doesn't make you actually cast the card. The spell must be actually cast paying the mana cost to be kickable.
Duel, i would assume that you can play the kicker off a YWill, as you are actually a casting it. With Dream Halls, I doubt it, and with Temporal Aperture (important when considering Searing Winds/Urza's Rage decks), I have no idea. Email some NetReps or something, as I don't know if kicker is completely covered by Oracle yet.