If the Thai citizen uses the second citizenship / passport
(for example Swedish that allow dual passports) on entry to Thailand, he/she will under the subject of law that apply to foreigners. If she / he does use
his Thai citizenship / passport when entering Thailand, she / he
same rights and obligations as other Thai nationals.

That might be very important if Thai person get in a legal problem.

Immigration No 2009.142/882 dated 26 Feb 2009. Guideline, Passport holder
with dependents or individual with 2 passport.