From the Shadows Rules pdf on the Support Page: (my quote fu is weak on these boards)

Any player can play | cards into Shadows by paying two gold during his or her marshalling phase. This is considered a player action. Playing any card into Shadows costs two gold, and there is no out of house gold penalty. The cost to play cards into Shadows can be reduced, but only by effects that specifically reduce the cost of playing a card into Shadows.

Cards that are in Shadows are not considered to be “in play,” nor are they considered to be “in hand.” They are considered to be “in Shadows.”

At the beginning of the any phase, a card that is in Shadows can come out of Shadows and into play at the discretion of the player controlling the card.

To clarify a rules question at our last tournament, a card can be " Played from Hand " (paid for in gold during the active player's part of marshalling phase before placing it on the table, if it's a Character, Attachment or Locaion; Costs paid and put down if it is an event, before going to the Discard Pile (some like the Epic Battles will be delayed in going to the Discard Pile, and Deathbound cards will go to the Dead Pile), " Put into Play " by some card effect (on the card itself (CS Khal Drogo) by event (To Be A Dragon) or card in play (Rhaenys's Hill)), or "Played into Shadows" (2 gold paid and the car placed face down on the table), correct? The only other way cards are going to leave your hand are by intrigue claim or a card effect discarding them (Confession), and the only additonal way they would come into play is " Coming out of Shadows " (by paying the non-shadows portion of the cost plus penalty at the beginning of the phase or by card effect (Ser Mandon Moore))

Queen's Knight reads:

House Targaryen only. Ambush.
After a non-Knight character comes into play, return Queen's Knight to its owner's hand.

The "comes into play" from his text covers all three of the above bolded entry points?

Unique cards can be marshalled into Shadows (by paying the two gold cost), even if that card’s owner already has a copy of that unique card in play or in his or her dead pile.

A unique card cannot be brought out of Shadows if another copy ofthat card is in its owner’s dead pile. If another copy of a unique card that you own and control is in play, you still may bring the card out of Shadows by paying its cost. However, the card immediately attaches to its copy as a duplicate.

Does this can mean "can only?" Rather, if my Shadows-crested Tyrion is in play, I can freely dupe him with a copy from the core set; Does a second Shadow-crested version have to pay 2 gold and wait in Shadows until the beginning of hte next phase, assuming I didn't have another card I wanted to bring out. I guess it could be more properly phrased, "Is attaching a duplicate count as playing a 0-cost card, or putting into play a card (in either case, by game rules)?" A "Shadows-crested cards cannot be marshalled directly into play".... Hrmm, maybe either of the above options count as "being marshalled, since that's the phase they happen in. I guess the "can" is tripping me up... having the option to do either is probably the coolest, but I'm curious.

Maester_LUke said:

The "comes into play" text covers any situation in which a card that starts out-of-play ends up in-play as a character. It doesn't matter if the card came from your hand, your discard pile, dead pile, Shadows or deck. It doesn't matter if the effect that puts the card into play is classified as "played," "put into play" or "comes out of Shadows." If the end result is a new character in play, the Queen's Knight is bounced.

I seriously think you might have over-thought all that. "Is there a card in play that wasn't in play at the beginning of the action or Response? And is it a character? Yes and yes? Then a character came into play. Good 'Knight,' so to speak." It's really that simple.

Maester_LUke said:

It does not mean "can only," although you are not the first person to think so. When you play a dupe from your hand (and remember, playing a dupe is simply playing a dupe), you are putting a card into play that is considered titleless, traitless and textless. In short, you have not Marshaled a card with the Shadows crest; you have played a dupe.

So yes, you can use a copy of a unique card that has the Shadows crest to dupe a copy that is already in play without going through Shadows first. Or you can marshal the card into Shadows and bring it out - or not - as defined in the Shadows rules with its points on how the action will jive with the rules for unique.