Why is everything doubled?

Question

Why does this give two Alices and two Bobs:
@startuml
Bob -> Alice : hello
@enduml

Answer 1

Do you mean that the Bob and Alice objects are also at the bottom of the graph?... 

This is prevented by using skinparam style strictuml

@startuml
skinparam style strictuml
Bob -> Alice: hello
@enduml

Regards

Answer 2

You can switch it off/on with  "hide/show footbox", e.g.

@startuml
hide footbox
Bob -> Alice : hello
@enduml