Should de-duplicate -isystem flags just like -I flags

The way I've setup my projects I get a lot of duplicate -I options passed to the compiler. This seems wrong, is it possible for the include_directories support to delete the duplicates?

Begin by tracing how include_directories handling produces compiler flags and compare it with the existing -I de-duplication behavior. Verify that equivalent -isystem flags are also emitted only once, while preserving distinct include directories.

Written by the indexing model from the issue text.