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Alyson Hannigan and Seth Green, “How I Met Your Mother” and "Buffy the Vampire Slayer"

"The Final Page, Part One And Two" -- Seth Green ("Buffy the Vampire Slayer", center) guest stars on HOW I MET YOUR MOTHER, Monday, Dec. 17 (8:00-9:00 PM, ET/PT) on the CBS Television Network. Pictured left to right: Jason Segel, Seth Green and Alyson Hannigan Photo: Ron P. Jaffe/FOX

INSET: BUFFY THE VAMPIRE SLAYER, (back row, from left): Anthony Stewart Head, Nicholas Brendon, James Marsters, Alyson Hannigan, Seth Green, (front): Marc Blucas, Sarah Michelle Gellar, (2000), 1997-2003. photo: Kwaku Alston / TM and Copyright © 20th Century Fox Film Corp. All rights reserved.

How I Met Your Mother Stills