Durant had 18.9 WS last year (second to LeBron) and barring injury or some sort of resting at year end here, should go at or above 18.9 this year and lead the league in Win Shares.
In the history of the game, only 4 guys have had consecutive seasons at or over 18.9 Win Shares:
1 -- Mikan 3
2 -- Kareem 3
3 -- Wilt 3 (two times) - Wilt was traded during 1964-65 and that season he had a very poor year for him, rolling in only 15 Win Shares. This cost him his potential streak of 7 straight, as he was 20+ from 1961-62 all of the way through 1967-68, except for this trade year.
4 -- Jordan - 4. He also holds the mark for oldest to put up such a year (1995-96, 32 years old)
LeBron has 2, but not consecutive.
What appears to be the case is that, for the truly greatest players, their best years are ages 24-28. Look at the list when you require 80 Win Shares and a .2 WS/48 during ages 24-28: http://bkref.com/tiny/nGPac
Total Win Shares - Wilt, Jordan, Kareem, Mikan, Oscar, LeBron.
WS/48 - Michael, LeBron, Kareem, Wilt, Mikan, Oscar.
The next guy on the list is David Robinson, and he has 9 WS less than LeBron.
So, Durant at 19 a year would end up at 95, trailing only Wilt, Jordan and Kareem, and at his current WS/48 pace of .298, he would surpass Jordan for #1. Needless to say, Durant is playing at a historically great level.