ResultParameter constructor generate invalid results for duplicate keys

This can be seen when adding parameters to a list of existing parameters.

var resultParameters1 = new ResultParameters(planRun.Parameters.Concat(new List<ResultParameter> {
                new ResultParameter("", "Duration", planRun.Duration.TotalSeconds),
                new ResultParameter("", "Verdict", planRun.Verdict)
            }));

This is not normally a problem, but the issue has been seen in some result listener implementations that relied on a different behavior.