Couldn't find a duplicate for this.

When placing blocks (especially half-slabs; occurs with other blocks as well however), the game can sometimes double-input and place two.

I've found this to occur when using both gamepad and keyboard/mouse input methods. I made sure it wasn't my input peripherals (mouse, keyboard, gamepad; drivers reinstalled, tried with other variants such as Xbox 360 controller vs. Xbox One controller, etc).