If you are using an instructor machine in a lab and run PowerPoint in presenter view it may keep the displays extended. To change to duplicate :
For Windows based machines:
Right click on an open area of the desktop and Display Settings.
- Under multiple displays, select "duplicate these displays", then click OK.
For Mac based machines:
- Go to the menu Apple menu in the upper left hand corner and select system preferences.
- Select Displays.
- Go to Arrangement and select "mirror displays" to start mirroring the display.