Why do they say to double wash hands, 20 seconds each to effectively get covid-19 off? is the number just to make sure the soap covers all the areas or does the soap need to be there for a fixed time for it to work? is double washing necessary?

COVID19 Hand Washing: Keeping hands clean is one of the most important steps we can take to avoid getting sick and spreading germs to others. Many diseases and conditions are spread by not washing hands with soap and clean, running water. Here is some useful information from the CDC that will answer your question: https://www.cdc.gov/handwashing/show-me-the-science-handwashing.html