In your case its pretty safe to assume that you have a dead MKR1000.
The double tap is timing critical and you could practice that on one of the working boards as you will see when it goes into bootloader mode. in device manager as it will come up with a new COM port.
Other than that you too may have a couple of dead ones.
N.B. To both of you make sure you have nothing else connected to the boards during testing.