Many single trigger designs include a bobweight to either block a "bounce fire" or switch the trigger to the other sear. I'm not familiar with the Repro single trigger mechanism, but I'd be willing to bet your problem has to do with the weight not returning when the gun is pointed up. That could conceiveably be from dried grease restricting movement as Kenny suggests.
Best to consult a gunsmith familiar with the Repro single trigger.
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