Weren't you in another episode?

Some actors have appeared as different characters in more than one episode of Supernatural. Generally these are actors who play minor roles, but of course even major stars of the show have appeared as different characters:

Jensen Ackles: Dean, Shapeshifter!Dean, Future!Dean, and Leviathan!Dean
Jared Padalecki: Sam, Meg, Lucifer, Gary, Leviathan!Sam and Gadreel
Misha Collins: Castiel, Jimmy Novak, Future!Castiel, Misha Collins, and Leviathan
Genevieve Padalecki: Ruby, Genevieve Padalecki
Matt Cohen: John Winchester, Michael
Katie Cassidy: Ruby, Lilith
Jake Abel: Adam Milligan, Ghoul, and Michael
Nicki Aycox: Meg The Demon and Meg Masters
Jim Beaver: Bobby and unnamed Demon
Lauren Tom: Linda Tran and Crowley

A number of characters - both angels and demons - have been played by multiple actors including: Azazel, John Winchester, Mary Winchester, Dean Winchester, Sam Winchester, Meg, Ruby, Crowley, Lucifer, Michael, and Raphael.

Minor actors who have appeared in multiple roles:

Roy Campsall played the wendigo in 1.02 Wendigo and an unidentified character in 1.10 Asylum.
Nico McEown played Lucas Barr in 1.03 Dead in the Water and a bully in 7.03 The Girl Next Door.
Daryl Shuttleworth played Chuck Lambert in 1.04 Phantom Traveler and Sheriff Carnegie in 5.05 Fallen Idols.
Michael Teigen played the teacher in 1.05 Bloody Mary and a police officer in 4.08 Wishful Thinking.
Anita Brown played Lindsay in 1.06 Skin and Hope Lynn Casey in 4.08 Wishful Thinking. She also had previously appeared as "Gill Girl" in an episode of Dark Angel alongside Jensen.
Christie Laing played Taylor in 1.07 Hookman and the Crossroads Demon in 2.08 Crossroad Blues.
Lara Gilchrist played Holly in 1.11 Scarecrow and the wraith in 5.11 Sam, Interrupted.
Leah Graham played Pauly in 1.11 Scarecrow and an unidentified character in 2.18 Hollywood Babylon.
Brent Stait played Scotty in 1.11 Scarecrow and Rick in 6.16 ...And Then There Were None.
Colin Lawrence played Jason in 1.12 Faith and Reggie Hull in 5.03 Free to Be You and Me.
Erica Carroll played a nurse in 1.12 Faith and a mother in 1.18 Something Wicked.
Ron Robinson played an unidentified character in 1.13 Route 666 and a possessed man in 3.12 Jus in Bello.
Shawn Reis played Lee Bender in 1.15 The Benders and an unidentified character in 1.12 Faith.
Mary Black played the old woman in 1.18 Something Wicked and Red Riding Hood's grandmother in 3.05 Bedtime Stories.
Linden Banks played Isaiah Merchant in 1.19 Provenance and Mr. Brennan in 6.09 Clap Your Hands If You Believe....
Jay Brazeau played the librarian in 1.19 Provenance and Dr. Corman in 5.14 My Bloody Valentine.
Christine Chatelain played Jenny in 1.20 Dead Man's Blood and Dr. Ellen Piccolo in 5.08 Changing Channels.
Terence Kelly played Daniel Elkins in 1.20 Dead Man's Blood and a doctor in 3.15 Time Is on My Side.
Carrie Fleming who played the dying nurse in 2.01 In My Time of Dying went on to play Karen Singer in 5.15 Dead Men Don't Wear Plaid and 7.10 Death's Door.
Keegan Connor Tracy played Karen Giles in 2.07 The Usual Suspects and Sera Siege in 4.18 The Monster at the End of This Book.
Aleks Paunovic played Boyden's friend in 2.08 Crossroad Blues and Skinwalker #2 in 6.08 All Dogs Go to Heaven.
Chilton Crane played Beverly Tanner in 2.09 Croatoan and Mary Lew in 8.06 Southern Comfort.
Tom McBeath played Mr. Carey in 2.10 Hunted and Jack in 9.07 Bad Boys.
Matreya Fedor played Tyler Thompson in 2.11 Playthings and an unidentified character in 2.15 Tall Tales.
Jonathan Bruce played Larry Williams in 2.11 Playthings and the convention host in 5.09 The Real Ghostbusters.
Roman Podhora played a lieutenant in 2.12 Nightshifter and the Sheriff in 6.19 Mommy Dearest.
Brad Dryborough played Glen in 2.17 Heart and Cal Hawkins in 5.05 Fallen Idols.
Garwin Sanford played Deacon in 2.19 Folsom Prison Blues and producer Jim Michaels in 6.15 The French Mistake.
Katya Virshilas played Lust in 3.01 The Magnificent Seven and Chastity in 5.03 Free to Be You and Me.
Martin Christopher played a doctor in 3.07 Fresh Blood and another doctor in 3.10 Dream a Little Dream of Me.
Emily Holmes played Mrs. Wallsh in 3.08 A Very Supernatural Christmas and Hester in 7.21 Reading Is Fundamental
John Shaw played Ben Waters in 3.14 Long-Distance Call and the hotel manager in 5.09 The Real Ghostbusters.
Daniel Arnold played a ghost in 4.02 Are You There, God? It's Me, Dean Winchester and Xavier in 5.07 The Curious Case of Dean Winchester.
Gerry Rousseau played Bill Gibson in 4.11 Family Remains and an unidentified character in 2.18 Hollywood Babylon.
Cainan Wiebe played Barry Cook in 4.13 After School Special and also had bit parts as a bully in 4.08 Wishful Thinking and a boy in 1.12 Faith.
Gabriel Carter played unidentified characters in 4.15 Death Takes a Holiday and 5.08 Changing Channels.
John Hainsworth played P. T. Sandover in 4.17 It's a Terrible Life and an unidentified character in 1.12 Faith.
John F. Parker played Mr. Stanley in 5.06 I Believe the Children Are Our Future and an unidentified character in 3.03 Bad Day at Black Rock.
Fiona Vroom played the nurse in 5.21 Two Minutes to Midnight and the dental assistant in 6.06 You Can't Handle the Truth.
Serinda Swan played Veritas in 6.06 You Can't Handle the Truth and a hospital receptionist who Dean flirts with in 1.21 Salvation.
Alexa Fast who played Missy Bender also played Emma in 7.13 The Slice Girls
Julia Rhodes who plays Marie McAnn in 7.18 Party On, Garth also appeared as a vampire in 6.05 Live Free or Twihard
Andrew Francis who plays the janitor Lee, son of Dale Lampert in 7.18 Party On, Garth played the actor Rick who worked alongside Tara Benchley in Hell Hazers II: The Reckoning in the episode 2.18 Hollywood Babylon.
David Stuart played the father of Evan in 2.02 Everybody Loves a Clown who was eaten by the Rakshasa and he played Pete in 7.20 The Girl with the Dungeons and Dragons Tattoo where he was eaten by a Leviathan
Ty Olsson played a vampire named Eli in 2.03 Bloodlust and is currently playing a vampire named Benny, who Dean meets in Purgatory.
Tyler Johnston who played Samandiriel also played Matt Pike, the bug obsessed kid in 1.08 Bugs.
Alex Diakun who played Mr Vili also played the Reaper in 1.12 Faith.
Aaron Pearl who played Jimmy Novak's possessed neighbor, Roger, in 4.20 The Rapture also played Officer Levitt in 8.03 Heartache.
Chris Nowland who played Chester Lew in 8.06 Southern Comfort also had an uncredited, unnamed role in 2.19 Folsom Prison Blues.
Brian Markinson who played Jerry Panowski in 1.04 Phantom Traveler later appears in 8.08 Hunteri Heroici as Stan Thompson, father of Sam's partner Amelia Richardson.
Maxine Miller who played Sheila in 8.08 Hunteri Heroici also played the old woman in the woods in 3.05 Bedtime Stories.
Kathleen Munroe who plays Benny's great granddaughter Elizabeth in 8.09 Citizen Fang also played Annette Doolittle, mother of Katie who is replaced by a Changeling in 3.02 The Kids Are Alright.
Chad Rook who played Desmond in 8.09 Citizen Fang also played Marshall Todd in 6.19 Mommy Dearest.
Vincent Gale who played Viggo in 8.10 Torn and Frayed also played Evan Hudson in 2.08 Crossroad Blues.
Andrea Brooks who played Maria/Gholandria in 8.11 LARP and the Real Girl also played Katie Burns in 2.06 No Exit.
Don Thompson who played the sheriff in 8.11 LARP and the Real Girl also played Mr. Guenther in 1.09 Home.
John De Santis who played the Golem in 8.13 Everybody Hates Hitler also played Freeman Daggett in 3.13 Ghostfacers.
Adrian Hough who plays Victor Rogers in 8.18 Freaks and Geeks also played Dr. Hydeker in 1.18 Something Wicked
Donnelly Rhodes who plays the older Father Simon in 8.22 Clip Show also appeared in 1.02 Wendigo as the retired ranger Shaw.
Artine Brown who plays Maurice the reaper in 9.03 I'm No Angel previously played a Night Watchman in 4.22 Lucifer Rising.
Forbes Anges who played Max Alexander in 9.05 Dog Dean Afternoon previoulsy played Foster in 3.03 Bad Day at Black Rock.

In addition to actors, some scenes have been re-used...

The aerial shot of the Impala traveling through green fields dotted with livestock was also used in 3.04 Sin City.
In 4.04 Metamorphosis, the original KAZ 2Y5 license plates are glimpsed on the Impala in a shot recycled from 2.03 Bloodlust. When Dean pulls over, the CNK 80Q3 plates have returned.
The scene of Sam and Dean burning Barry's bones in 4.13 After School Special is a reversed shot from 2.18 Hollywood Babylon.