Réponse "intésessante" (et légèrement comique même) à cette question trouvée dans
"
Phuket Gazette" :--
The Thai government does not allow its citizens to hold dual nationalities, except if the
other country agrees to grant citizenship to that Thai person, in which case he/she can
use that as well.
If he/she uses the other citizenship to reside in
Thailand, then they will be subject to the
laws as they apply to foreigners.
If he/she uses the Thai citizenship, they have the same rights as normal Thai citizens.
A Thai consular official--
En gros, la "double nationalité" est refusée sauf si... la personne possède aussi la
nationalité d'un autre pays

Typiquement gentillet et thaïlandais, pas vraiment de quoi s'inquiéter !