How I can read Parent - Child DataSet into object structure (If it possible)

Hello. I'm beginner and don't know fuctionality of this great component.
Can someone help me with reading data to object model

    public class Order
    {
        public int OrderId { get; set; }
        public DateTime Date { get; set; }
        public List<Detail> Details { get; set; }
    }
    public class Detail
    {
        public int OrderId { get; set; }
        public string Product { get; set; }
    }

SELECT O.OrderId, O.DateTime, D.Product
FROM Orders O
INNER JOIN Details D ON D.OrderId=O.OrderId

May be I should do so?
var x = multi.Read<Order, Detail, Order>((o, d) => { o.Details.Add(d); return o; }, splitOn:"OrderId");
But it returns in collection duplicate records

Review the Order and Detail models, joined SQL query, and proposed multi.Read<Order, Detail, Order> call with splitOn:"OrderId". Start with Dapper's multi-mapping documentation and examples; done means clarifying how the joined rows should map into the object structure and why duplicate records appear.