Ok it's not technically a double tax but...
Here's just a thought to get people talking. If you purchase an iTunes card with a value of say $50, you will pay sales tax on that as well. Being from the Midwest, I know that everyone already here wants to tax anything plastic as well under a guise of "green" since it lasts forever in a landfill. So you may end up paying a tax on that as well.
Now the average rate for sales tax in the Midwest is about 7.25 depending on where you are. So let's start doing a little math.
$50 for the card, plus 3.63 for sales tax. Plus if there's a green tax they tend to cost about 1% so that's an extra 50 cents. Out the door roughly $54.13 for that card. Now when I use it, or if I give it to someone else as a gift and they use it in Wisconsin they'll pay the 5% per download. Let's just assume $1 for each download, and we have each song costing $1.05. You get about 47 and a half songs with that gift card I just gave you there. Wisconsin did nothing for the upkeep of any of the infrastructure to get me that download. Probably did nothing to help the artist in anyway, and odds are have done nothing in general to said person to listen to the music (other then telling them to turn it down while in the car!)
I guess IMO states, nations, etc that tax the transaction to purchase something more then once annoy the crap out of me and really are just saying "WE NEED YOU TO FUND US IN EVERY WAY. YOU'LL PAY OR ELSE!"