You would want to double the size of the "finished" size of the pieces and then add the 1/2" seam allowance to them.
Example...your original pattern says to cut a 4 1/2" block (4" finished), to double it it would be 8" + the 1/2" seam allowance to 8 1/2"
Double-ling it to 9 inches would not work out correctly, IMHO