Prevent scheduling of duplicate proof attempts by run_external_proof.
Prevent scheduling of duplicate proof attempts by run_external_proof.

If the user happens to select all the goals in the IDE and press replay,
a quadratic number of proof attempts are actually scheduled: each goal is
scheduled as many times as it has ancestors. This patch fixes it.

ssaavedra/why3,ssaavedra/why3,jpdeplaix/why3,douglas-larocca/why3,jpdeplaix/why3,jpdeplaix/why3,douglas-larocca/why3,ssaavedra/why3,douglas-larocca/why3