It's fairly common in Mac keyboard layouts that, if you type a dead key twice,
the second key press will both produce a non-dead character and also
perpetuate the dead-key state.  For example, with the U.S. layout, Option-E,
E will produce "é" and Option-E, Option-E, E will produce "´é".  Windows
keyboard layouts don't tend to do this.  The second key press produces the
non-dead character and clears the dead-key state.