C# Linq - Find Duplicate value in list and select it's id
<p>Here is my Linqpad example:</p>

<pre><code> void Main()
{
    List&lt;test&gt; test=new List&lt;test&gt;();
    test.Add(new test("001","2016/10/5","11:00","A1"));
    test.Add(new test("002","2016/10/5","11:00","A1"));
    test.Add(new test("003","2016/10/4","11:00","cc")); 
    test.Add(new test("004","2016/10/4","10:00","cc"));
    test.Add(new test("005","2016/10/3","11:00","cc"));

    var list=test.GroupBy(x =&gt; new { x.BOOKING_DATE, x.PERIOD_NAME,x.BOOKING_TABLE})
    .Where(g=&gt;g.Count()&gt;1).Select(G=&gt;G.Key.BOOKING_TABLE);

    list.Dump();
}

public class test{
    public string ORDERM_ID;
    public string BOOKING_DATE;
    public string PERIOD_NAME;
    public string BOOKING_TABLE;
    public test(string id,string date,string period,string table){
        this.ORDERM_ID=id;
        this.BOOKING_DATE=date;
        this.PERIOD_NAME=period;
        this.BOOKING_TABLE=table;
    }
}
</code></pre>

<p>I want group by BOOKING_DATE and PERIOD_NAME
to find duplicate BOOKING_TABLE.</p>

<p>In this example ,I find the duplicate value is "A1",
But I want select the ORDERM_ID in list.
How can I build a linq query to do this?</p>