Someone asked this question on a message board:
I think the second nomination is humongous because it t shows you're not just a one-hit wonder. I feel like 2-time noms like Brad Pitt, Rob Downey Jr, Joaquin Pheonix, Samantha Morton, Amy Adams*, Catherine Keener and now Matt Damon are now members of the club of established actors because they have two and if you look at their filmographies, it felt like they were being increasingly recognized for notable work around the time they earned their second nomination. I even feel like the above list of names is more established than a one-time winner like Rachel Weicz, Jennifer Hudson, Halle Berry, Helen Hunt, or Cuba Gooding Jr, because they've shown they can reach the threshold of Oscar nomination twice.
As a result, I think it's not hard to get a second nomination if you've deserved it but I think a voter would think twice if you're not a one-hit wonder (someone who had a really good role just once). I can't imagine someone like Eddie Murphy, Dan Akroyd, Sandra Locke, Casey Affleck, Sophie Okendo, even Alan Alda, getting a second nomination if they suddenly did a great film tomorrow unless it was amazing like Charlize Theron in monster. It would fall a little under the well they already have a nom rule.
To look at this further:
-Brad Pitt got nominated in Benjamin Buttons after turning in notable leading performances in Assassination of Jesse James and Babel. Babel was significant because he almost got nominated for it and the picture got nominated for an Oscar and won the golden globe that year.
-Catherine Keener got nomination #2 for Capote after she had one of those prolific years that people take note of: She was also in 40-Year Old Virgin and Interpreter and won the NBR prize for all three
-Rob Downey Jr. had the upcoming lead in Soloist (this was a little bit of reverse dues because the film was scheduled to be released in 08 and if the trailer doesn't make your heart melt alone, I don't know what would). Downey Jr. also had just become Tobey Maguire 2.0 by hemming a megablockbuster in Iron Man. Three years prior he garnered some note in Kiss Kiss Bang Bang and Good Night and Good Luck
-Matt Damon already had an Oscar in writing and acting but a decade later it was clear that he was one of the most respected and prolific actors in Hollywood and needed more to show for it. Bourne Ultimatum, Good Shephard, Syrianna and Departed were all memorable turns (he got zero Oscar buzz for the latter two because George Clooney and Leo DiCaprio, Jack Nicholson and Mark Wahlberg outshone him for the Oscar buzz). Even his role in All the Pretty Horses could have made a decent case for an Oscar nom. By 2009 with fading buzz for Informant, his best role to date, the voters needed to apologize to him by sticking him in the supporting category for a role that was (at least, in my opinion) pretty forgettable.
-Amy Adams filmography might be a little short compared to others on the list but she only started acting this decade and wasn't a commodity until 2005. She had been in a consistent number of pictures sup to 2008 where she has shown range and expanded her resume: She did a comedy with Will Ferrell, a Mike Nicholls picture with Hanks and Roberts, starred in a Disneyesque flick, did another indie in Sunshine Clearing, and shined in a theatrical piece in Doubt.