As a season ticket holder myself, this has been a festering sore that I've complained about for many years, both here and privately to the Iron Dukes. It's not just the prevalence of opposing fans in the upstairs seats at Cameron, which they obviously obtained from season ticket holders through online resellers. We encounter an even more infuriating situation at the ACC Tournament and the Final Four, where we routinely arrive at our assigned seats (as Priority Level 2 Iron Dukes for two decades) only to discover that a substantial portion of the more desirable seats in the Duke section are occupied by UNC fans or others wearing the colors and cheering for Duke's opponents.
When I've complained about the issue, I've been told that there is nothing Duke can do because the problem is that those people get their seats from online resellers who receive them through corporate season ticket purchasers who give tens of thousands each year to enjoy Level 1 priority. So they can give or sell their seats to anyone with impunity. While I appreciate and welcome Jack's message, the simple fact is that the Iron Dukes can't have it both ways -- if they want to accept the big bucks from corporate donors, the Iron Dukes and the rest of us have to accept the consequences, which include the frustration of seeing seats in Cameron and at tournaments occupied by non-Duke fans.
When the people who control those seats have no inherent loyalty to Duke and can sell them for many times their face value or use them for valuable business promotions, there's no real incentive to turn them in to the Iron Dukes for a small credit. And unless the Iron Dukes are willing to lose those large annual contributions, there's no practical means of enforcing the rule. It sucks, but it's probably irremediable.