Minecraft shows on both monitors when I go full-screen

I did not do anything for this to happen, it just happened one day when I started up Minecraft. When I went into full-screen mode, my Minecraft showed up on both my monitors.
However, I'm only able to move my mouse on my main monitor (on the right in the pictures - the cursor is invisible), while the buttons of the Minecraft menu only react to the mouse movement on my second monitor.

• I have tried switching the main monitor, but it doesn't help.
• I have also tried to go into full-screen mode in the settings, but the same happens, except my main monitor goes black instead.

It's the same on all versions of Minecraft.

The image below shows what happens when I go into the settings and turn on full-screen. You can see I have my mouse on the right monitor on "Multiplayer", but it shows up only on the left monitor:

And the following image is when I press F11, and, as you can see, the right monitor has just gone completely black but I can still only use the mouse on the main monitor:

For me, when Minecraft enters full screen mode, the other monitor fills with junk. A workaround I found is to play in windowed mode: press F11 to switch to 'small window' and then the 'square' on the title bar to expand the window to fill the screen.

It still leaves the title bar and the frame around the window but Minecraft stops messing up other windows, so I can use a video reference or a wiki page, or discord chat on the other screen.