Data Validation for Duplicate Entry

Good Morning !

can anyone solve this issue...

Sr.No SUN
10 A
12 C
10 B
12 D
10 A
12

Here is the question that if i put "A" in front of 10 and again if i put 'A' in front of 10. how it will stop me as showing "Duplicate Data".
I hope you all understand my problem. Please give solution ASAP.

Thank you !...