Doppelganger

Can Doppelganger copy an insane character?
No. Doppelganger reads, “When you
play Doppelganger from your hand
choose a non-unique character in play.
X is equal to the printed cost of that
character.” and insane characters have no
printed cost there is no way to determine
the cost to play Doppelganger. Thus,
Doppelganger cannot copy a an insane
character.

How does Doppelgänger (The Yuggoth Contract
F79) work when copying a character with an
ability that triggers when it enters play, like
Focused Art Student (The Yuggoth Contract
F8)?
Doppelgänger reads, “When you play
Doppelgänger from your hand, choose a
non-unique character in play with printed
cost X or lower. Doppelgänger becomes
a printed copy of that character.”
So, in this example, if there was a Focused
Art Student in play and I wanted to play
Doppelgänger and copy the Focused Art
Student (which has a printed cost of 2), I
would drain a domain with 2 resources
(1 of which was Yog-Sothoth because it
is still a Yog-Sothoth card at this point),
and choose the Focused Art Student to
copy. The card then enters play as the
Focused Art Student, and the triggered
ability, “Response: After Focused Art
Student enters play, draw 1 card.” will
trigger. Note that Doppelgänger ignores
loyalty and steadfast of the character
it copies because its copying effect
takes place after costs are paid and
those restrictions are checked. Once
the Doppelganger has copied a character,
it remains as a copy of that character
(including cost, faction, icons, skill,
triggered abilities and subtypes) until it
leaves play. If Doppelgänger goes insane,
it is treated as though it was the card it
copied gone insane. This means that it
still retains the card title of the copied
card, even though the rest of the card is
still treated the same as a normal insane
character.

(2.11) “X” (The Letter “X”)
Unless specified by a preceding card,
card effect, or granted player choice, the
letter “X” is always equal to zero.
For example, Darrin controls Shadow
Company (Summons of the Deep F89),
which has an “X” for the card’s skill
value. His opponent, Tommy, targets it
with Called By Azathoth (Summons of the
Deep F9). Since the Shadow Company has
a blank text box, it does not have a value
assigned to X. Thus, its skill is now zero.

You know, I haven't used Doppelgänger for a while but early on when I started playing it seemed pretty popular.  I especially liked having a way to get extra copies of characters that benefit from multiple copies in play or discounting characters like Servant of Glaaki.

The best use I've seen for it was in a two-faction Mi-Go deck. It's a good card, but in recent years, it was superseded by other great cards on the restricted list - at least for me.

I can see how the effects in a Mi-Go deck could be amplified quite nicely.

I assume the Doppelganger would benefit from Resilient if it copied a Baka (or similar Resilient non-unique character).  The Doppelganger would be a copy of the Baka until it leaves play.  Rules on Resilient read "when the Resilient character WOULD leave play."  The Resilient would seem to take effect before the Doppelganger has left play, and thus, before the Doppelganger has ceased to be a copy of the Baka.