How to copy repeated numbers

Does anybody know how to copy a repeated number from one cell to another.
Example:
I have a list of numbers (1,2,3,4,5,6,1,2,3,7,1,2) on cells A1 thru A12. As you can see numbers 1,2 are repeated on two ocassions and 3 is repeated once. How can I "automatically" populate cell B1 with the repeated number 1, cell B2 with the repeated number 2 and cell B3 with the number 3. Also, "automatically" populate cell C1 with the second repeated number 1 and cell C2 with the second repeated number 2.
So, basically I just want to put in numbers in column A and "automatically" populate the other columns as the numbers repeat. Thank you.

See attachment for example.