Thanks for the report and sorry about that.
This is fixed in last beta http://beta.plantuml.net/plantuml.jar
I don't agree with the fact that it is not possible to include a file twice.
Yes, this was a bad decision taken a long time ago.
Unfortunately, it's difficult to change the behaviour of !include because it would impact many existing PlantUML diagrams.
As workaround, you can use !include_many that has the same behaviour as #include C-version.
Maybe we will introduce a new !include_once feature that will act like the actual !include implementation.
Then, in some hypothetical future, we could switch the actual behaviour of !include from !include_once to !include_many
But this may be confusing for users.
What do you think about it ?