Hearthpwn

If you take this discovery, READ THE CARD TEXT before you cast. Spells can either hit

1. Any character, which means that Frostbolt might turn around and hit your face. If you're going to cast spells like this, don't make it the first spell you cast in your turn (unless last resort) so it doesn't trigger Double Time. 

2. Enemy Minions, so casting that Breath of Sindragosa will be helpful if there are several minions, but useless if you just want to kill one low-health one as the repeated cast won't hurt the enemy's face even after the minion is dead. 

3. Enemy Characters, so casting Cinderstorm will hit both the enemy minions and their face. If you're building a deck with Double Time, cards like Cinderstorm, Arcane Missiles, and Greater Arcane Missiles are what you're gonna want to get. 

Also worth noting is how this interacts with spells that cause you to Discover cards from your deck. After you choose which cards to discover, Double Time will pick a random eligible card and put it in your hand. Ergo, if you use Brass Lantern to discover 3 cards of your choosing, Double Time kicks in and discovers 3 additional random cards from your deck, giving you a total of 6 new cards in your hand: 3 of your choosing and 3 random. 

I've only been playing Hearthstone for about a year. Going through the 'Monster Hunt' with this card selected as Toki has taught me to ALWAYS look at the exact text of a damage spell. 'Deal X damage' can be as bad for you as for the enemy.  It's fun to ponder the different strategies it takes to make this play correctly.

If you grab 2, the first spell you cast each round is casted 2 more times, at random targets (including yourself)

What happens when you take 2 of them?

The spell repeats twice. So a Cinder Storm that normally launches five cinders for 5 damage, will instead shoot FIFTEEN cinders for 15 damage. 

Mana wyrm does not get the benefit of the second casting of the spell. Not sure if this is a bug or not.

so if you have double "Double Time" the first spell plays thrice