First, note that existence of anything isthere is at most 2 potato headsnotclaimed. If some world is completely devoid of potato heads (as a result of war or pestilence...), then this statement is true in that world. The statement claims that the existence of three or more potato heads is impossible.

Therefore, an equivalent, but, perhaps, less intuitive, was to say this is: It is not the case that there exist three p.h.'sand all of them are different. Note the last part because the formula is true in a world with just one potato head.

For the second statement, as a hint, look at the definition of the uniqueness quantifier.