Autofill Bug?

Can someone help me with this problem - is there a bug with excel or is it something in the set up of my excel?

In A1 type a name (or random word), in B1 type the following formula "=SUMIF($A$5:$A$10,A1,$B$5:$B$10)

Copy the two cells down to row 4 - you can leave the names the same or change them so you have all different names.

So you have a column of names and a column of formula

then add another name to the bottom of the list in cell A5 and hit return

the formula in cell B4 will automatically copy down and create a circular reference.

How do I stop this?