Av mode doesn't make it take two photos, so it must be something else.

If you'd turned on bracketing, then I'd think it would take three, not two, so that's probably not it.

One possibility would be that you have 'burst mode' enabled, and it's simply getting two shots in before you release the shutter. Does the "drive mode" icon (above the number of shots available at the bottom right) look like a rectangle, or a stack of three rectangles? If it's the three, then you have burst mode on. Use the 'left' button (to the left of 'set') to change it to single.

Beyond that, does it actually take two shots, or does it just sound like it? Sometimes, mirror lockup or pre-flash measurements can make it sound different.

That's it, burst mode was on.  Thanks Andy

That is a compliment to your trigger shooting capabilities. Many folks 'stab' at it. A good slow squeeze (if burst mode is on) would almost always give you at least 2 shots.
I'm willing to bet at least one, if not both, of your shots are nice and sharp with good technique like that.
Glad you got it sorted out though!