I expect that McCutchen will also be helped by having played much better than was expected. While there have been a number of repeat MVP winners, over the years voters have also often given the award to the player who seemed to "come out of nowhere" as compared to the player who just turned in another typical as expected season.
For example, Jason Giambi was named MVP in 2000 as his A's won their first title in several seasons. He had a big year, hitting .333 with 43 HRs, 137 RBI, a .647 SLG and leading the league with a .476 OBP. He only ranked 5th in the AL with a 7.4 WAR, well behind league leader Pedro Martinez's 11.4.
The next season Giambi lead the AL with a 8.8 WAR, and his raw numbers were pretty much the same as 2000: .342 38 120, with AL leading OBP of .477 and SLG of .660. He didn't repeat as MVP, as the "new kid in town" was Ichiro Suzuki took the award even though he was only 5th in WAR.
In 1962 Willie Mays lead the NL in WAR with 10.2, and in HRs with 49 to go with a .304 average and 141 RBI, but the MVP was given to Maury Wills, who ranked 9th in WAR with 5.8; Wills was the new big thing, stealing 104 bases. He wasn't nearly as good as Mays but he got the award.
Roberto Clemente partisans made a big deal prior to 1966 about Clemente being underrated and it later paid off for Roberto as he was given the 1966 MVP. Yes, he hit for more power than before with 29 HRs, but he also ranked 6th in WAR with 8.1, behind Mays who had a WAR of 8.8 and my own MVP pick, Sandy Koufax, whose 9.5 WAR tied for the NL lead with Juan Marichal.
Barry Bonds won 7 MVP awards in his career*, but he arguably was wrongly denied the 1991 MVP. He followed up his 1990 MVP season with another big year, ranking 2nd in the NL with a 7.6 WAR (Tom Glavine lead the NL but pitchers rarely are given the award). The media named Terry Pendleton MVP. Pendleton's WAR ranked 5th with a 5.8 score, but he "came out of nowhere," a former unknown who won the NL batting title for the division champ Braves.