Revert "Don't allow duplicate entry in the completion popup."

This reverts commit 53462d915fcd4ae805d69eb6ff7ba104d390a5a1.

That was causing issues when snippets were disabled.
Refactor the compiler arguments processing
Remove a workaround due to broken cindex.py iterator
debug mode: Print clang command line for parsing
debug mode: Print clang command line for completion