Why can I see an unknown monitor "VGA-1-2" in my display setting?

[TrunderBird]

My notebook has two graphic card(Geforce 630M and Intel embeded). And I have installed Mint Linux 18.2 cinnamon edition into my mobile hard disk as a live-linux system. When I have started up the os,all the thing are right except that the screen refresh is abnormal.,when I moved the mouse-cursor quickly,I could see lots of duplicated mouse pointers. Then I launched the dIsplay setting,To my surprise I had seen two monitor:one was my notebook's LCD screen, and another displayed as "Unknow monitor (VGA-1-2)",when I closed this monitor in display setting (switch off the button in the display setting)', the system worked perfectly.My question is:
What is this "Unknown monitor"?And how can I delete it permanantly?
Below is the result of "xrandr"

Screen 0: minimum 8 x 8, current 1366 x 768, maximum 32767 x 32767
LVDS1 connected primary 1366x768+0+0 (normal left inverted right x axis y axis) 344mm x 193mm
1366x768 60.01*+
1360x768 59.80 59.96
1280x720 60.00
1024x768 60.00
1024x576 60.00
960x540 60.00
800x600 60.32 56.25
864x486 60.00
640x480 59.94
720x405 60.00
680x384 60.00
640x360 60.00
DP1 disconnected (normal left inverted right x axis y axis)
HDMI1 disconnected (normal left inverted right x axis y axis)
VGA1 disconnected (normal left inverted right x axis y axis)
VIRTUAL1 disconnected (normal left inverted right x axis y axis)

VGA-1-2 connected
1024x768 60.00
800x600 60.32 56.25
848x480 60.00
640x480 59.94
1024x768 (0x43) 65.000MHz -HSync -VSync
h: width 1024 start 1048 end 1184 total 1344 skew 0 clock 48.36KHz
v: height 768 start 771 end 777 total 806 clock 60.00Hz
800x600 (0x44) 40.000MHz +HSync +VSync
h: width 800 start 840 end 968 total 1056 skew 0 clock 37.88KHz
v: height 600 start 601 end 605 total 628 clock 60.32Hz
800x600 (0x45) 36.000MHz +HSync +VSync
h: width 800 start 824 end 896 total 1024 skew 0 clock 35.16KHz
v: height 600 start 601 end 603 total 625 clock 56.25Hz
640x480 (0x47) 25.175MHz -HSync -VSync
h: width 640 start 656 end 752 total 800 skew 0 clock 31.47KHz
v: height 480 start 490 end 492 total 525 clock 59.94Hz




The italic formats was added by me,This is the information of the "Unknow monitor".
Nee you help.Thank you !