I would be absolutely livid.
Yes, there is a "No-Double-Upgrade' policy at United when you are using miles or certificates. I have always contended that it is in place in order to protect the infamous "Employee Class".
But what happened to you is quite different. You were given this upgrade in "compensation" for a problem. Double upgrades occur for a variety of reasons. Sometime UA is only too happy to give you a double upgrade when it allows them to sit one additional person on the plane.
In this instance, you were given this upgrade at the discretion of the first gate agent, in compensation for a major irregularity that strongly impacted you. It was not a mistake. It was a customer service judgment call from a UA "owner-employee" who deemed it justifiable. and authorized it. The second gate agent stole this upgrade from you for his/her friends. That is not justifiable, and I would do everything I can to get this second agent in trouble while praising the first one.
It is completely unacceptable, and If I were you, I would write a polite letter to UA explaining the circumstances and your strong disagreement with the second gate agent's actions, and asking if it is UA policy to kick paying passengers out of a legally obtained (albeit upgraded twice) seat for the benefit of a gate agent's friends. Had I been you, I would have asked for a supervisor on the spot.
I'd be quite upset about it.
Such employees give UA a bad name.
[This message has been edited by Droneklax (edited 01-14-2001).]