is $Canvas.user.id or $vnd.instructure.User.uuid unique across all the instances ?

[JavierHex]

I am using the LTI variable substitution $Canvas.user.id and the $vnd.instructure.User.uuid and I am wondering if these ids are unique just for the Canvas instance or they are unique for whole Canvas? is it possible to get the same  $Canvas.user.id value on different Canvas instances ?

Thanks 

[ColinMurtaugh]

$Canvas.user.id is only unique within the specific Canvas instance; $vnd.instructure.User.uuid should be globally unique. 

--Colin