Advice. Yes......be careful. It is an addictive sport.
Seriously, though, I would not (and I have not) worry about shooting Double Trap until I had a bunch of 25 straights under my belt and had done a fair amount of handicap shooting.
What about the "fast" machine? To what are you referring? If you are shooting ATA 16 yard Trap singles, the birds are all at the same speed.
Olympic/International/Bunker Trap machines throw a much faster bird but there are fifteen machines in the trap house.
If you were shooting Olympic Trap and shot 21 on your first outing, you did very well indeed.
"Only hunting and mountain climbing are sports. The rest are just games." - R.Ruark
NRA Life Member