[isabelle] "Duplicate session"

The command

    ~$ ./bin/isabelle build -d src/Tools/isac/ -v -b Isac

creates the message

    Duplicate session "Isac" (file "src/Tools/isac/ROOT")

but no session.
How can the session be created ?


PS(1): these are all respective sessions found in the user space:
    ~/.isabelle$ find -name Isac

PS(2): the session should be created from an "unauthorized Isabelle version" derived from Isabelle2013-1; the previous session created from Isabelle2013 is in "./heaps/polyml-5.5.0_x86-linux/"

PS(3): shouldn't the new session be built into "./heaps/polyml-5.5.1_x86-linux/" ?

PS(4): deleting "./heaps/polyml-5.5.0_x86-linux/Isac" doesn't help (as expected from (3))

PS (5): Isabelle2013-1 finds these logics:
    ~~$ ./bin/isabelle findlogics
    HOL Pure

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