Is there anything in the AMD specification to say that require'd modules must be supplied with the same object? It seems to be fairly common practice to assume that a require'd module is a single instance supplied to all requiring modules, but is there anything to prevent a module loader treating loaded modules as merely cached (possibly reloading them at some point)?
For example (hypothetically speaking), would an AMD loader be guaranteed to distribute the same instance of a message bus module across various different dependent modules, so they could use it to send each other messages?
Yes, modules should be singletons.
From the spec:
define() function
...
factory
The third argument, factory, is a function that should be executed to instantiate the module or an object. If the factory is a function it should only be executed once. If the factory argument is an object, that object should be assigned as the exported value of the module.