Ubuntu does not see monitors and duplicates them.

Good thing you added the xrandr output. From that bit of output, it is clear that we could not get the size of the gamma for the default output.
What does this mean? For some reason, there is no way to change the screen resolution to the maximum supported resolution of your monitor. This situation is possible in a Linux system, when installing two or more monitors, there may be several reasons.
The screen shows what you are trying to do through the GUI, but it limits you to a screen resolution of 800х600 . If you want to set the resolution higher or lower than what you have, then, unfortunately, nothing will work out in this way. Next, back to xrandr . This utility has a few more features than the graphical interface.
To view the current system settings, run the utility without parameters:

 $ xrandr 

Since you do not need permission, you need to add it manually. To do this, you first need to convert a regular entry to the VESA format. Use the cvt utility for this purpose cvt

 $ sudo cvt 1366 768 60 

Your screen resolution and frequency should match on both screens (i.e. available in their specifications). We copy from the output of the utility everything that is written after Modeline . Next, use the copied text in xrandr :

 $ xrandr --newmode "1366x768_60.00" и далее весь вывод до конца 

Next you need to add the created permission to the list of available ones for the screen we need:

 $ xrandr --addmode [ваш способ подключения, к примеру через: HDMI-1] 1366x768_60.00 

To check the way monitors are connected:

 $ xrandr --listmonitors 

Then you just need to activate:

 $ xrandr --output HDMI-1 --mode 1366x768_60.00 

We start again xrandr and check the parameters. If everything went well, then you need to save the changes in the configuration file ~/.xprofile

 $ nano ~/.xprofile 

To clarify any information on the system there is a good utility inxi , if you don’t have it on your PC, install:

 $ sudo apt install inxi 

For example, you need information about installed video cards:

 $ inxi -G 

You can also collect information in this way:

 $ sudo lshw -C display 

References:

xrandr: Failed to get size of gamma for output default on Unix & Linux
Xrandr: Failed to get size for gamma for output on Linux Mint Forum
xrandr - Linux man page
manpages.ubuntu