You won't find any text or law dealing with double taxation. If the OP is from a country with a DTA and has paid tax elsewhere, there's no tax to pay in Thailand. Unless of course someone is stupid enough to volunteer to pay tax, and I think many expats already fit this category. Thailand taxation is self-assessment, too easy
But his Singapore Tax Residency may take precedence over Thai Tax Residency. The DTA provides the mechanism to decide which prevails. It's more to do with residency rather than where tax is paid. Read the DTA. The same applies in Australia. I have dual tax residency but am only required to pay tax in Australia, and I am not required to submit a tax return in Thailand. His situation might be the same, I'm not saying it is, but it's certainly worth exploring