MS Access database problem

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C#

hi.
I have question and hope that somebody will help me to solve this problem.

for example there are textboxt , datetimepicker, datagridview and MS Access baza in my project

I have two Colums:

Column Account        Column Datetime

4                      02/02/2011                    (true)
1                      03/02/2011                    (true)
4 (adding second time) 04/05/2011 (in the same year)   i want to get error  (False)
4                      01/01/2012                (true)

And it is normal when I add "4" to database first time... I want to take "error" if i want to add "4" second time with the same year. I want one account number in one year..

Thanks in advance....
hope you will help me.