Sometimes 1+1 is 2, like when you're counting stuff.

Sometimes 1+1 is 1, like when you just need a Boolean indicator of whether something is true. Pressing the elevator button multiple times should behave the same way as pressing the elevator button once. Planning out a delivery route requires a stop at every place with at least one item to be delivered, but the route itself doesn't change when a second or third item is added to that stop.

Sometimes 1+1 is 0, like when dealing with certain types of rotations, toggle switches, etc. Doing a 180° rotation twice is the same as doing it zero times. Same with doing a reflection transformation twice.

A good engineer understands the scope of what they're doing, and its limits.