Hi Arthur,
I'm sorry to hear that your Duet WiFi has a fault. We ship a lot of Duet WiFis and unfortunately a small number of them pass all our tests (including of course the stepper driver tests) but subsequently suffer an infant mortality failure of a stepper driver chip. This is not completely surprising, because we ship boards using several thousands of these chips every month.
If it is a genuine UK-manufactured Duet, it is covered by the warranty that we provide; in which case, please contact your supplier and refer them to this response for authorisation to replace your Duet under warranty.