Originally Posted by mctheriot
I have the "show due items" icon active in the Menu bar. When I click it and then select a context, it opens a new instance of OF rather than just bringing up my selection in OF already open.
By "new instance", I assume you mean a new window, not a new instance of the application. There should still be only one OmniFocus process running (you can check with /Applications/Utilities/Activity Monitor).
Anyway, yes, that is the expected behavior. I think the idea is that opening a context from the Dock contextual menu or from the menu bar should not mess up your view of the current OmniFocus window, so a new window is opened.
I actually really like this behavior. I often have my main window focused on a particular folder in planning mode or a specific group of contexts selected in context mode. It would be really annoying to lose those settings every time I wanted to quickly check a different context by selecting something from the Dock menu or the menu bar.
When I'm done with the second window, I just hit Command-W to close it, leaving only my main window open.
If it's still a problem for you, maybe open a feature request to add a preference setting to control whether those contexts open in a new window or not? Go to Help->Send Feedback in the menu bar to make an official request.