Support for specifying a stack_name separate from name was added in #551 to support stacks with the same name, but in different regions/accounts, however, this introduces the possibility for two nodes in the graph to operate on the same stack.

I think it'd be wise to add an extra layer of safety so that stacker will error out if two nodes in the graph operate on the same stack ARN, as this would almost always be programmer error (e.g. copy/paste but forget to change stack_name).