This cannot be done for anywhere near your $8b. You need many times that amount of money just to handle some of the problems.

If you are willing to miss some people this is likely the best solution possible. Establish a very large NGO with offices in every country in the world. You'll need enough people to cover the entire planet at the same time - and then setup desks where everyone can come in, get their dollar, and get their left hand stamped with an ink that cannot be washed off in less than a day. This is what countries do to prevent duplicate votes.

And this won't get everyone, but it should prevent most duplications.