Hi,
Not sure if you mean you are seeing duplicate displays or the same thing on your external monitor, but either way, you can just go to the settings to choose if you want your external monitor to duplicate or 'extend' (show a different screen or second screen) your notebook screen.
There are shortcuts and different ways to do the same thing, but since Microsoft's website has information about it, will just provide their link below if you want to take a look.
https://support.microsoft.com/en-us/help/4340331/windows-10-set-up-dual-monitors=====================
As far as your external monitor resolution goes, you may want to check the specs to make sure it is not a settings problem, or if you refer to the link above for Microsoft's website, you can see in the picture that there is a setting to adjust your monitor resolution in the same area if you want to check it.
It's not mentioned or seen on that page, but you should see pictures of both your monitors in that same area as well and you can left-click either picture of the monitor to change the settings for each corresponding display.