Since these are BBCLASSEXTEND operations which are usually done last, if a user manually inherits them, together with other classes (like packagegroup for example), the behavior might not be what the user wanted (an allarch package instead of two SDK_ARCH dependent packages, for instance). This might lead to unnecessary rebuilds and warnings for duplicate files, etc.

So, we'd better warn the user about the correct behavior and, if he/she really wants this, we should provide a means to acknowledge this in order to silence the warning.