A common resource (e.g. Patient) could be used in multiple events, e.g. Patient-change, ImageStudy-change, and DiagnosticReport(for labs)-change. Currently the subscribers are responsible for subscribing to events that are relevant to their workflow. If a DI viewlet subscribes to two events Patient-change and ImageStudy-change, and Labs viewlet subscribes to Patient-change and DiagnosticReport(for labs)-change, it seems to be legitimate because the DI viewlet is not interested in labs related events, and vise versa for the Labs viewlet. However there is a risk that the Labs viewlet won't be notified when the user switches the patient in the DI viewlet via ImageStudy-change event, therefore the patient would be out of synch.
Currently there is no guidelines/rules defined around how handle such scenarios in FHIRcast.