InclusionDirectives.Add throws on duplicate directive

[firesgc]

In commit a8325ce, calling

rootContainer.InclusionDirectives.Add(inclusionDirective);

throws an exception if the directive is already present in the collection.

Expected Behavior:
Adding the same InclusionDirective multiple times should be ignored or handled gracefully.

Actual Behavior:
An exception is thrown when the directive already exists.

Thank you very much for this awesome library!