We can tweak the way properties are merged via PropertyOptions item.
Several usecases:
1. Remove duplicates from cpp.includePaths, cpp.defines
2. Merge scalar bool values using OR/AND (e.g. 2 modules set bool hasFeature to different values)
3. Select maximum/minimum for int values/versions.
4. Smart selecting for e.g. cpp.cxxLanguageVersion.

This can be achieved via some predefined algorithms or via a custom function. Maybe we can use the map/reduce idiom for properties?