[prove] duplicated warnings
$ why3find prove
Warning: prover alt-ergo@2.5.3 not found (why3)
Warning: prover alt-ergo@2.5.3 not configured (project)
Warning: prover alt-ergo@2.5.3 not found (why3)
Warning: prover alt-ergo@2.5.3 not configured (project)
Warning: prover alt-ergo@2.5.3 not found (why3)
Warning: prover alt-ergo@2.5.3 not configured (project)
...