When you finish painting a wall do you paint a second layer?

Applying a second coat of paint is not necessary unless you are still able to see the original paint under the coat you just applied. If you were able to achieve an even, uniform coverage with the first coat then pat yourself on the back and admire your good painting work. Similar colors tend to cover with one coat. Trying to cover a dark shade of paint like brown, green or blue with something much lighter like antique white will invariably require 2 full coats for adequate coverage.