If you get an error like this:

It means: the Singleton class SOMECLASSNAME has multiple instances.

This means that, for some reason, there is more than one object of the class that is marked as Singleton—and there should, by definition, only be one.

To fix this, start the MDriven OCL Debugger towards the same database that has the error.

Select all the objects of the given class: SOMECLASSNAME.allinstances, which is easiest via a Seeker (1 & 2) and Search (3), then delete all but 1 of the objects (4 & 5).