Labs tend to be loud or chaotic places.
It's easy to space out for a few second and forget if you've added a component.
Here's a bench tip to help you stay on track: Line up your components in the order you need them. After adding one, move the tube back one row so you don't have to remember if you've already used it.
With this system, even if you lose focus for a few seconds, you'll remain on track. It takes a some time and practice to build this habit but you'll be glad you did when you can rely on muscle memory at the research bench.