You might be worried about people entering dummy, junk email addresses just to get your freebie.
Most autoresponders (I use Getresponse) will auto-check that an email is valid before adding it to your list.
And even for those that don't - as long as you set up your opt in process in the right way, you can overcome this.
The best way is not to deliver your promised-freebie right on the thank you page (this is open to abuse, and gets lower email response).
The best way is to show people, on the thank you page, how they can find your freebie in their email inbox.
This way, they can only get it, if they actually enter a valid email address.
This is, itself, a very cool way to get people to confirm their intent without sending them those pointless and annoying "confirm your email" emails...
What's more, using double opt in is not a smart move because;