UFC officials announced the fight Monday afternoon.
Duke (2-0, 1 no-contest) lost via unanimous decision to Raquel Pennington in the quarterfinals on the show, while Morgan (2-0) lost via submission to Sarah Moras in the elimination round as well.
In the main event, Gray Maynard and Nate Diaz will square off in a lightweight contest.
Here's how the TUF 18 Finale fight card looks now:
Gray Maynard vs. Nate Diaz
Julianna Pena vs. TBA
Chris Holdsworth vs. David Grant
Rani Yahya vs. Tom Niinimaki
Jared Rosholt vs. Walt Harris
Sean Spencer vs. Drew Dober
Ryan Benoit vs. Josh Sampo