How to check Duplicate value in Data table's Rows?

In data table In three rows value given below

 

1	2	3	4	5	6	7	8	9	10	11	12	13	14	15	16	17	18	19	20	21	22

23	24	25	26	27	28	29	30	31	32	33	34	35	36	37	38	39	40	41	42	43	44

45

46

47

48

49

50

51

52

53

54

55

56

57

58

59

60

61

62

63

64

65

66

 

 

I will check duplicate value occur. like if 2 is occur in two times show the validation message.like Duplicate value occured.

if (ds.Tables[s].Rows[0][i].ToString().Trim() == "")
{
dt.Rows.Add();
dt.Rows[mn][0] = "Tag Number Should Be Mandatory" +ds.Tables[s].TableName; 

}

Above code i will check if the empty filed occur in table show the mandatory message.Now i want to validate duplicate value occur.