Did the Bitcoin fork resulting in the Bitcoin Cash created money from nothing? Since you can double-spend your old Bitcoin coins and the Bitcon Cash coins, independently---one on each side of the fork---it seems somehow that money was created for free. Isn't this a problem? What prevents people from creating more and more forks to the point of undermining the crypto-currencies value?
To answer the questions in the body of your posting:

Spending BTC "on one side of the fork" and BCH on the other cannot be called "double spending".
"Money created for free" is a misnomer. Yes, you have twice as many coins after a fork, but the combined value of both coins "right and left of the blockchain" is initially the same as just before the split, namely the coin with the former name has 100% and the one with the new ticker 0%. But from that point on price discovery on the market (exchanges) will determine the value of each strand individually. 

"What prevents people from creating forks . . .?"
Among others the risk of loosing trust and competitiveness in regard to other crypto currencies. 
If nevertheless a fork is triggered by some actors in a respective community, there must be important reasons why it couldn't be avoided. A fork is a declaration of war, after all efforts to compromise and reaching consensus failed. 
But as we can see in case of Bitcoin, a fork does not necessarily undermine the value of that currency. The combined value of Bitcoin and Bitcoin Cash is now, 3 weeks after the fork, considerably higher than before.