This is supposed to prevent double visiting.

But as the code stands now it was no-op.
I think this situation has since been rectified.

... with CpsBodyInvoker.createBodyBlockNode
[FIXED JENKINS-26074]

I fixed the problem by making FlowHead removal automatic. We keep
FlowHead around until another FlowHead subsumes it.

In this way, FlowHead that belongs to completed CpsThread will hang
around for parallel steps until a big StepEndNode comes in and
designates everyone else as the parent.