c standards with regard to pointer behavior

If i have two pointers to structures a and b, like this:

Thing *a, *b;

I can copy the contents of the Thing pointed to by b to a like so:

*a = *b;

(which is basically the same as memcpy (a, b, sizeof(*a))...)

So the question is, what if a and b both point to exact same location?  Is the behavior undefined, or should it leave the contents unchanged as you would expect?

I can test it easily, but i want to know if it is guaranteed portable.