Making espresso correctly - rerunning hot water several times through an espresso puck to fill an Americano
Most coffee shops use about 20 grams (18-24 grams in my experience) for a double shot of coffee, and 12-16 grams for a single shot. If you see a cafe using 20 grams for a single shot, its only because they are using double ristrettos (2 half-shots of espresso) instead of a single shot. Some people prefer the flavor profile of a double ristretto.
As you pull hot water through your shots, you extract different compounds from the coffee. Most people stop their shots when they've extracted all the tasty compounds, and I'll usually try to stop my shots before the coffee starts to taste bitter.

If you prefer your coffee a bit more bitter and full-bodied, as well as with whatever other flavors you're extracting at the end feel free to pull extra shots from the same coffee, however I would normally reccomend just adding extra shots to the americano instead.

All that being said, Barista Champion Matt Perger reccomended during his world barista run a process similar to what you're doing. He reccomended using a coarser grind and running the shots for way longer to achieve a coffee similar to a filter coffee or americano. However he also has the benefit of expensive professional machines that are highly tuned, so your experience may vary.


Summary

My reccomendation if you are going to pull the shots more than once is to use a coarser grind for your coffee. Otherwise feel free to pull the shots as long as you like until you dont like the taste. You could also try pulling the separate shots into separate cups to taste them before mixing them together.