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I'm running out of ideas to debug this, maybe Antoine or Amaury can help :-)
One last idea (not sure it will work though):
If Channel's finalizer gets called twice, inside Channel.__del__, you
could save a string representation of the current backtrace as an
attribute, and print the previous and current backtrace is there is
already one such attribute (i.e. print the backtrace if __del__ is
called twice).
There may be false positives (for example a finalizer resurecting the
object would end up in __del__ being called twice), but it might yield
useful info.
You might also want to save the thread ID and time.
But this is really strange...