If you're having trouble using duet, check below to see the most frequent questions users have. And if you don't find your answer, please feel free to contact us and we'll help sort out any issue.
Does your Mac ask you to trust this computer on every connection? This issue is caused by Apple's trust this computer bug. Apple's suggested fix is to restart your Mac and try again. More info about about the bug from Apple's support:http://support.apple.com/en-us/HT202778
Many applications (including iTunes and XCode) are affected by this bug :(
Please make sure you have the latest Mac App from http://duetdisplay.com/mac
Sometimes duet can't connect the first time, but unplugging and then replugging the USB should work.