don't confuse "double coin" with "double money". you are doing the first not the second.

for example if you buy $1000 worth of bitcoin, after any fork you will have:
- $1000 worth of bitcoin (which will most probably be worth more, $1100 for example).
- and $50 worth of the forked altcoin.

(note that above numbers are based on the recent fork of bitcoin called bitcoin cash and its price)