samtee That happens when you have more than one phone connected to the computer, or if you also have an emulator running. If you can't disconnect the other device, Run adb devices to see what's connected, you then can run adb -s and then the serial number that corresponds to your phone before the command, for example adb -s abcdef install yourpackage.apk (or if the other device is an emulator you can do -d)
If it's recognizing more than one device when it shouldn't, you can try adb kill-server, and restarting adb
