Unfortunately the RadDiagram doesn't have a built-in workflow validator so you'll have to do this manually. There are different ways to validate your diagram and it all depends on your custom scenario - are you using MVVM, custom shapes, custom connectors and etc.
By design the diagram control is highly customizable and you can change a lot of its default behaviors - I've attached a sample project that has a runtime connection validation - you can start a connection only from a right connector and you can end it only in a left connector, also each connector can have only one connection.
I hope I was able to help you and if you have more questions feel free to ask.
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