INSTALL_FAILED_UPDATE_INCOMPATIBLE: Package signatures do not match the previously installed version; ignoring

Just in case if somehow your old application is not removed 100% or its data is not removed.

Try:

adb uninstall "com.yourapp.yourapp"

I met the same error today.And I found thishttp://blog.bihe0832.com/android-v2-issue.html.It's in Chinese.The key points describe below:

After package your apk file, unzip it,check the file META-INF/ANY-NAME.SF.I sign the apk with jarsigner command line tool.I found that the alias putted after the jar-file when exec the jarsigner, will used as the ANY-NAME.Then open the ANY-NAME.SF, if you found a line of X-Android-APK-Signed: 2,That's the cause of the error.Resign your apk,if the line of code gone, the apk will be installed success.

Make sure you know which device adb is dealing with.

I had multiple devices available. I couldn't understand why uninstalling on my emulator didn't get rid of the message. It turned out that my hardware phone was plugged into the computer to charge, and also had USB debugging enabled which was confusing adb.

Disabling USB debugging on my hardware phone, and uninstalling the app on the emulator got it sorted.