DataTable dt = new DataTable();
            dt.Columns.Add("a");
            dt.Columns.Add("b");
            dt.Columns.Add("c");
            dt.Columns.Add("d");

            dt.LoadDataRow(new object[] { "qq", "ss", "dd", "fg" },false);
            dt.LoadDataRow(new object[] { "qq", "ss", "dd", "reb" }, false);
            dt.LoadDataRow(new object[] { "qq", "acd", "dd", "tbg" }, false);

            var res = dt.AsEnumerable().GroupBy(p => p["a"].ToString() + p["b"].ToString() + p["c"].ToString());

------解决方案--------------------
C# code

void Main()
{
            DataTable dt = new DataTable();
            dt.Columns.Add("a");
            dt.Columns.Add("b");
            dt.Columns.Add("c");
            dt.Columns.Add("d");

            dt.LoadDataRow(new object[] { "qq", "ss", "dd", "fg" },false);
            dt.LoadDataRow(new object[] { "qq", "ss", "dd", "reb" }, false);
            dt.LoadDataRow(new object[] { "qq", "acd", "dd", "tbg" }, false);
        var query= dt.AsEnumerable().GroupBy(d=> new {a=d.Field<string>("a"),b=d.Field<string>("b"),c=d.Field<string>("c")})
                            .Where(g=>g.Count()>1);
        Console.WriteLine(query.Count()>0?"有重复":"无重复");
}

------解决方案--------------------
代码类似,只不过你这是字符串
参考http://topic.csdn.net/u/20120412/15/b36be23a-c01e-49b3-a011-3ca1b2acc68b.html

------解决方案--------------------
C# code

            DataTable dt = new DataTable("test");
            dt.Columns.Add(new DataColumn("a", typeof(string)));
            dt.Columns.Add(new DataColumn("b", typeof(string)));
            dt.Columns.Add(new DataColumn("c", typeof(string)));
            dt.Columns.Add(new DataColumn("d", typeof(string)));

            dt.Rows.Add("qq", "ss", "dd", "fg");
            dt.Rows.Add("qq", "ss", "dd", "reb");
            dt.Rows.Add("qq", "acd", "dd", "fg");

            var result = dt.AsEnumerable().GroupBy(r => new { a = r["a"].ToString(), b = r["b"].ToString(), c = r["c"].ToString() }).Where(g => g.Count() > 1).Count() > 0;            
            Console.WriteLine("result: " + result.ToString());
            Console.ReadKey();

------解决方案--------------------
 DataTable dt = new DataTable();
           dt.Columns.Add("a",typeof(string));
           dt.Columns.Add("b",typeof(string));
           dt.Columns.Add("c",typeof(string));
           dt.Columns.Add("d",typeof(string));
           dt.LoadDataRow(new object[] { "qq", "ss", "dd", "fg" }, false);