Several issues were reported in our Windows Phone SDKs (both for WP 7.5 and 8) and we are releasing a quick fix. New versions 2.7.0.3 and 2.8.0.3 address an issue when ArgumentException was occasionally thrown by the ad control. Another issue with control trying to load an ad even when collapsed was fixed.
As usual the new controls are available via nuget.
AdDuplex blog 