The following code (complete minimal project attached) produces no errors even though it attempts to register a C++ singleton and a QML singleton under the same import name:
The only indication that something is wrong (this is part of a much larger application) is that the logging category name is printed as "undefined", since no such property exists in the C++ singleton.
I would expect that an error is produced when trying to import the "logging" QML singleton. E.g.: