So Doppelganger basically works in the same pattern as ambush except it costs more and instead of disrupting your opponent's hand (maybe removing big threats) it allows you to maybe catch up with them (often grabbing cards that are only really useful for their deck). I assume I would see it more-- or much at all-- in non-Hidden games, but for a sorta-durable card it's current use rating of "0.54" is still really dang low.
(Oh, I know I maybe should be focusing on the other least-used cards since this is in 10th place and not first, but this is one whose problem I knew how to say succinctly.)
Oh, and current card details;
Common
Illusion
0/5/0(B/G/R)
263 / 4902 Played
430 / 6618 Discarded
792 / 13000 Drawn
(Also I don't know exactly what those last six numbers mean because I would expect the first/second pair to be a ratio of the times it's been played/discarded relative to when it could have been played/discarded but that doesn't match with "drawn".)
Effect
If targeting self and not New, summons the matching card, else replace a card in hand with self
if (!isset($mynewflags[$cardpos]) AND $mode == $cardpos)
$nextcard = $hisdata->Hand[$cardpos];
else
{
$mydata->Hand[$mode] = $card->id();
$mydata->NewCards[$mode] = 1;
}