If you're having trouble using Duet, check below to see the most frequent questions users have – if you don't find your answer, please feel free to contact us at firstname.lastname@example.org and we'll help sort out any issue.
Does your Mac ask you to trust this computer on every connection even though you have already selected "Trust This Computer"? If so, this issue is caused by a bug on Apple's end. Apple's suggested fix is to restart your Mac and try again. Here's more info about about the bug from Apple's support: http://support.apple.com/en-us/HT202778. Many applications (including iTunes and XCode) are also 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.