During the FxA device registration process, if the server returns a device session conflict error, we have to make a 2nd request to FxA's /account/devices to identify the conflicting device.  Soon, they will include in the 1st request error response the conflicting device ID, which would allow us to save a round trip to the server.  We should update our code to use that new returned field.  http://searchfox.org/mozilla-central/source/services/fxaccounts/FxAccounts.jsm#1725  https://github.com/mozilla/fxa-auth-server/pull/1990
Fwiw, this change will be in FxA train 92.
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