1. You try to install the apk on your device and you receive the error
INSTALL_FAILED_ALREADY_EXISTS
2. You look in your program list(where your apps and widgets are listed) and you cannot see your app listed. So, it appears it does not 'already exist'.
Solution
1. On your Android device, go to Settings - Apps(or Applications) - All(or Manager Applications)Click on the Application you are trying to install.
Click Uninstall button, to uninstall.
2. It MAY say that it did NOT successfully uninstall. However, I found that it did actually uninstall it.
3. Try to install your Android app again.
