prohibit duplicate source files in enabled groups

Having the same source files in more than one enabled group is
considered an error. There's no proper way to define conflict
resolution rules.

Task-number: QBS-593
Change-Id: I93d25b7e3484ce3a4da4e10299e1a3bf10951ca2
Reviewed-by: Christian Kandeler <christian.kandeler@digia.com>