[FIX] don't assign produced preconditions twice
[FIX] don't assign produced preconditions twice

When a Precondition is produced the parse_produce_precondition will call
parse_precondition for each generated precondition. This call leads to
the new preconditions being assigned to the testrun. When
parse_produce_precondition returns it will also try to assign the new
preconditions. This is wrong and now fixed.

csdb/Tapper-MCP,tapper/Tapper-MCP,csdb/Tapper-MCP,tapper/Tapper-MCP