you mean you already got charged for it twice and the beta is not showing up?
1. contact stardock to return one of the two purchases (explain what happened)
2. go to impulse, manually add in the registration number you received with your purchase (it does not happen automatically for beta products)
3. click show pre-release versions.
be warned that the beta is actually an alpha, half the game has not been implemented yet, there are 1 or 2 world spells and no combat spells yet, there are no heroes, the game crashes and is buggy, etc... this is for very early stage input on things... wait until beta 3 if you want a finished but unpolished game.
however, if you just messed up placing the order and were NOT charged yet, you should place a preorder and wait. it will get you in on the next wave.