you can get the 15k package at no velocity if you want and since your a prior customer will will get 10% on top of that which will make it 16500.
As far as when you might hit the same domain twice, that could be as soon as you submit your order honestly. The last time we counted, which was several updates ago, we had way over 40k+ unique domains ( 70k+ when you add in subdomains ) and over 3.4M urls.
Since our service randomly picks a url out of the bunch, there is a chance that it picks 2 or more urls from the same domain during the run of your order.
However, most of our customers have more then 1 site in their order ( multiple domains and/or multiple urls ) and our service also picks those up at random too during submission. The chance you would have one of your multiple sites/urls on the same url as was posted on before within your order are very very slim and the chance it would be on the same domain is rather small too. However, I can't fully say it wouldn't never happen, and if it did I would for sure get you some more links to help diversify if you point it out to me.
Based on all that, depending on how many sites/urls you send us, the size of your order and the length of your order ( dripped out ) its very hard to pinpoint a % for you. Throw in the randomness and it gets even more hairy.
I can tell you none of our customers have complained about such a problem.