If UNC and Duke would have the same record in ACC play at the end of the season, and if they split their head-to-head meetings, the next tiebreaker would be each's record against the 3rd place team. As of now, that team is FSU, which beat Duke in their only matchup.
If UNC were to lose to FSU they would likely be 12-3 in ACC play (assuming they win tonight) going into the Duke game.
If Duke beats Clemson they would be 13-2 in ACC play going into the UNC game.
In this scenario Duke would have to beat UNC to receive the #1 seed since if both teams were to finish 13-3, UNC would have the tiebreaker based on their 1 win versus FSU against Duke's 0 wins against FSU.
If VT were to end up passing FSU for 3rd place, UNC would still hold the tiebreaker because they beat VT in their only matchup.
Unless I am not understanding the tiebreaker rules.
http://www.theacc.com/sports/m-baskb...022607aaa.html