The 30-year-old journeyman started the final round four shots behind 54-hole leader Brendon Todd. Duncan snuck into a playoff with Simpson at 19-under par thanks to a flurry of late birdies, including back-to-back ones to cap his fourth-round 65.
Duncan drilled a 12-foot birdie putt in the second go-around on No. 18 at Sea Island Resort's Seaside Course. Simpson had a par putt remaining but it didn't matter.
The victory marks Duncan's first title on any tour since 2014. He played 158 worldwide events prior to breaking through at the RSM Classic.
Duncan nearly doubles his career earnings with a $1.188-million winner's prize.
Todd was unsuccessful in his attempt to win three straight tournaments. He fired a 2-over 72 after entering Sunday's round with a two-shot edge.
Here's how the final leaderboard looked at the last official event of 2019:
|Place||Player||To Par||R4 Score|
*Denotes playoff win