DynamicParameters.Add cannot 'override' the property of template if value is DbString

[jiabiao]

I am using an object as parameters template, and use the DynamicParameters.Add method to 'override' a string property to DbString so that it can use SQL Server index.
But i got the exception : The variable name has already been declared.

        public static void Run()
        {
            Test(1);  //ok
            Test("1"); //ok
            Test(DateTime.Now); //ok
            Test(new DbString() { Value = "1" });  //System.Data.SqlClient.SqlException: 'The variable name '@Value' has already been declared.
        }

        public static void Test<T>(T value)
        {
            using (SqlConnection conn = new SqlConnection("Data Source=.;Initial Catalog=master;Persist Security Info=True;"))
            {
                var template = new { Value = value };
                var dp = new DynamicParameters(template);
                dp.Add("Value", value);
                conn.Execute("SELECT 1 WHERE @Value IS NOT NULL", dp);  
            }
        }

[jiabiao]

Not only DbString, any type implement SqlMapper.ICustomQueryParameter have this issue.

For other types(does not ICustomQueryParameter), there are some code to prevent dupilcate parameters, so there are no problem.
DynamicParameters.AddParameters code:

if (isCustomQueryParameter)
{
// DbString goes to here.
}
else
{
                    bool add = !command.Parameters.Contains(name);
                    IDbDataParameter p;
                    if (add)
                    {
                        p = command.CreateParameter();
                        p.ParameterName = name;
                    }
                    else
                    {
                        p = (IDbDataParameter)command.Parameters[name];
                    }
}

so ICustomQueryParameter will be appended to command twice:

1. IL DynamicMethod: appender(command, template);
2. SqlMapper.ICustomQueryParameter.AddParameter(command, name)