How to copy/sync music downloaded from Apple Music on iTunes to iPhone? I have Apple Music subscription. When I make songs available offline(on iPhone 5s) I can listen to them on the device that I downloaded the songs without internet and not offline on iTunes on my Desktop running Windows 10. 

If i make music available offline on computer then I can listen to them only on computer without internet and not on my iPhone. I have to re-download the music onto my iPhone.

I'm using the same apple account. All my songs are getting downloaded twice. What should I do to copy the music from Desktop to iPhone without redownloading?

Hardware


iPhone 5S - iOS 9.1 
Windows 10 - 64 bit


Why should I have to download a song on each device when I'm signed in on the same Apple account?

Also, when I add a song to Apple Music, It comes under my music from my iPhone and also to itunes my music in computer, but i have to make the music available offline again even if i did download on to the phone earlier. The offline music does not sync.
You can't sync Apple Music downloaded content from iTunes to your iPhone (or the other way round). Each device has to download it separately (as of iOS 11.3 and iTunes 12.7.3).