I believe Thailand has a tax treaty with the US that prevents double taxation. I receive tax credits in the US for all interest income earned here in Thailand. The money is withheld at source at a rate of 15%, but my marginal rate in the US is much higher than that, so I wind up paying 15% to Thailand and say 21% to the IRS. With FACTA it is difficult to cheat.
I man I know, claimed to have gotten a rebate from the local Thai tax office on all his withheld taxes. He seemed legit and encouraged me to do the same, but I am lazy with bureaucratic things, so I never checked it out.