It's pretty common. Based on radar data about 10-15% of Near Earth asteroids (larger than 200 meters) are contact binaries (or contact-binary shaped at least). We don't have enough data on main belt asteroids, trojan asteroids, and especially TNOs/KBOs to say what the percentages look like for those populations but it's a fair likelihood that the proportions are at least as high if not higher among KBOs.

14 missions to comets, 20 to asteroids, and around 50 planetary small moons imaged well enough to see their shapes. Under a hundred miles in diameter many have irregular shapes. I would guess this shape only occurs a few percent of the time.

Wednesdays news conference speculated UT shape is primordial, while the others may have been carved through four billion years of planetary evolution.