cannot understand the deduplication property

[macit-emre]

when i use this style as inpu :
h1{ color:red!important; } h1{ color:blue; }
$parser->parse() method not gives me the line with !important, it gives this output
h1{color:blue}
as we see, code with !important should stay but it gives the non-important line in the compressed output, can you help me?