Warn, if two contexts have the same name.
Currently, we have the following issues:
ContextChecker::processElement: "data/syntax/cgis.xml" context duplicate: "Region Marker"
ContextChecker::processElement: "data/syntax/cgis.xml" context duplicate: "Commentar 1"
ContextChecker::processElement: "data/syntax/cgis.xml" context duplicate: "Commentar 2"
ContextChecker::processElement: "data/syntax/diff.xml" context duplicate: "File"
ContextChecker::processElement: "data/syntax/objectivecpp.xml" context duplicate: "Preprocessor"
ContextChecker::processElement: "data/syntax/spice.xml" context duplicate: "String"