Dunno, never duplicated my dual monitors. Windows will mirror, it's just a click, but a duplicate screen could be different than a gaming primary screen since only the primary allows input, the secondary is just a duplicate. So it could either just be a video/cartoon type output or could be a full gaming output, I'm unsure.
The downgrade from 4k is 1080p, 1440p has a different pixel count, it's quad-HD, 4k is quad-full HD, there's a difference, HD is 720p and full HD is 1080p.
It's 2:07AM here currently or I'd go see for myself, but the Mrs is asleep.