Doppelgangers (Season 5, Episode 24)
They call them Doppelgangers
How I Met Your Mother - 2005 Doppelgangers 5-24 was released on: USA: 24 May 2010 Australia: 26 August 2010 Hungary: 8 November 2010 Germany: 23 February 2011 Finland: 11 April 2012 Japan: 9 December 2014
at birth
No
imdb.com
Judy Greer plays Royce in the episode "The Wedding Bride" of How I Met Your Mother.
It was called "Intervention"
Amanda Peet
That is Stacy Keibler who played the role of Karina in episode 100 of How I Met Your Mother. She has appeared in numerous other shows as well such as Chuck.
Season 5, episode 17 - Of Course
It was Season 5, episode 12, Girls vs. Suits.
The last episode of How I Met Your Mother has not aired yet. The producing team have made mention that the next season, 8 may be their last season.