@Greg **********
this is completely incorrect. Most countries have a double tax treaty and each one is unique. But generally speaking the way it works is that you owe taxes in BOTH countries, but your foreign tax payment can be used as a tax credit to Thailand.
So for example if you sell some stock in the US and pay 10% capital gains taxes, the Thai tax on that might be 30% (the brackets are much lower) at which point you would owe the difference, 20%, to Thailand.