The idea seems fine, but have that cost three energy makes it wasteful in most situations, unless you have it attached to a weak Pokemon. There are a lot of attacks for three or even two energy that, in most circumstances, deal 50 or even more damage. Having to basically sit out one turn in order to double (or maybe not even double) the damage on the next turn for such a high energy cost seems ridiculous.
Take Keldeo for instance. If it already has three energy attached to it, at the very minimum it is doing 50 damage per turn. Assuming they are all water energy, it's doing 110 per turn. Using an item like this wouldn't be beneficial, it would be detrimental. This applies to a lot of attacks for the same energy cost. You would either break even or lose out. The only point at which I see this being useful is if your opponent is immune to damage on the turn in which you use the Double-Up Bag, which would allow you to make up for at least some of the missed damage on your next turn. Otherwise, as it stands, this card is only useful when attached to very weak Pokemon. The circumstances under which it would actually be beneficial are so rare that it's not in a million years worth giving up your Computer Search/Dowsing Machine/Gold Potion/whichever Ace Spec you currently use.
In order to make it more applicable, something would have to be changed on it. Making the energy cost lower is one. That way you can use it while you build up energy to a stronger attack. Another would be to simply add the damage to every single turn, without having to use an attack and then wait a round. Doing this might even make a lower damage addition, like 30 or 40, worth it.
As the card is, I like the concept, but the execution is poor.
