want to check while entering number is duplicated with previous entered numbers

hope someone can help me.
I want to check while entering number is duplicated with previous entered numbers or not.
I mean,
I want to enter 1 to 600 in Column A randomly. Eg., 23,345,223,5,... From cell A1 to A600.

Column A
23
345
223
5

I first enter 23 in A1 then I enter 345 in A2, then 223 in A3, then 5 in A4, etc...
While I'm entering 345 in A2, I want to check whether there is 345 in previous cell of column A, ie., A1.
So also, while I'm entering 223 in A3, I want to check whether there is 223 in previous cell of column A, ie., A1 and A2.
So also, while I'm entering 5 in A4, I want to check whether there is 5 in previous cell of column A, ie., A1, A2 and A3 ...etc.
From A1 to A600, I want to check, is there any duplicated number in previous cell or not.

I will very appreciated to who help me.

Thanks in advanced.

Wiki Winn

Perhaps you want to think about using Data Validation ?

Based on your question highlight range A1:A600 -> Data -> Validation:

Set to Custom: =COUNTIF($A$1:$A1,$A1)=1
Set Error Msg to: Duplicate

However what happens if you enter 10 into A10 and then enter 10 into A2... based on your example this would not generate an error as A2 would only look to A1:A2 for duplicates, and A10 will not be tested - when entered it was the sole instance of 10.

Perhaps you want to test the entire range at all times in which case revise the Custom validation to:

Set to Custom: =COUNTIF($A$1:$A$600,$A1)=1

you could use conditional format
for a2 and copied to all cells in range
=AND(A1<>"",A2=A1)
edit wrong that only tests the cell immediatly above! doh
try
=COUNTIF($A$1:A2,A2)>1