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The actual wording in the documentation is: "This must be
called at most once per thread object." To me this implies
that the effect of calling start() twice is not defined. 

There is an assert statement in the implementation of
Thread.start(), but - like all assert statements - its
behaviour is dependent upon the value of the builtin
__debug__ variable.

Looking through the rest of the threading module, the
documentation could be tightened up, particularly with
respect to which behaviours are and aren't defined.