Worry not fellow Aardvark.Media enthusiast! This is a bug in the diffgenerator leading to an invalid file. It happens when you use the same namespace in two different files (which is completely valid and actually the reason namespaces exist) and both files have Types with the attribute [<DomainType>] in them. For the incremental magic to work, the diffgenerator creates a module named Mutable for the two files, and these modules happen to be in the same namespace ... hence the conflict. The only way to avoid this is to either put all domain types in one of the two files or use a different namespace in one of the files