I am attempting to create a new COM object in my DLL using the ATL Simple Object Wizard in Visual Studio 2005.
I enter the short name for the object
I never found out why the wizard determined that the object name existed already - I'm guessing something was cached somewhere that I couldn't track down.
In the end, I appended a 2 to the end of the interface name when creating it which allowed it to be added. Then I replaced all the occurrences of IXxxInterfaceName2 with IXxxInterfaceName. Everything worked after this.
If I ever track down the root cause of this problem, I'll update the answer.