Preventing duplicate entry of a row in Sheet 2 while copying selected rows from Sheet 1

I have checkboxes placed at the end of each row on Sheet1. The rows which have checboxes selected, are copied to another sheet(Sheet2) on click of a button. However, I want to prevent the same row being inserted again if i check the same row next time. I have a column with unique values which are placed in first column, and which i can use to check the duplicate entry of a row. I'll take the following example, the rows containing in Sheet2 (though i have more number of columns than what is shown here):

ColA Col B
A 2
B 4
C 5


Here ColA consists of unique values. Now, next time if i want to select and copy the following Rows from Sheet1, only Q 1 and J 4 should be copied and A 2 shouldn't get copied (since it is already present in Sheet2):

ColA Col B
Q 1
A 2
J 4

Please let me know what is the best way to achieve this?