Following steps to reproduce:

Create an app called xyz

Create cluster for this app, specify NONE of the additional naming attributes

Create second cluster for this app, giving a dev-phase such as 'stage'
Asgard will tell me: There is already an auto scaling group named xyz

But: If I try to create a second cluster with the 'stack' or 'free-form detail' set to some value, Asgard has no issues and creates the group.

So in this case (starting out with an ASG that has no additional attributes set) and then creating a second one, there is an inconsistency in the behavior of 'dev-phase' as compared with other attributes.