When I add a relationship between two elements in a diagram (in this case a communication diagram) in Sparx EA it adds it to every diagram that has those two elements. This means I have to go back and remove it from every other diagram that has those two elements, a task that gets harder the more diagrams you have.
Is there a way to stop this happening?
Not going to discuss the meaning of what you are trying but you can select the connector after creation and from the context menu use Visibility/Hide in other diagrams.
You can also write an add-in that can do that automatically for you. In that case you need to subscribe to EAOnPostNewConnector.