Indeed, I will if required, but obviously rather wouldn't. Not sure if the announcement in the Gazette will be enough or if they need the letter of Naturalization or ID card(which takes a bit longer), my immigration office has never done this before.
when applying for Thai citizenship, I had to get a form from the UK embassy, stating that I would renounce my British citizenship on getting Thai citizenship. However, the consul told me that there was no need.
Technically, dual citizenship isn't allowed but it is a grey area in the Nationality Act. Naturalized citizens could get their Thai citizenship taken away if they are found to have used their former nationality in Thailand.
Firstly, the ministry of labor don't inform immigration. I stopped work once and handed in the WP, as there was 11 months left on the visa, I didn't go to immigration. When getting a new wp a couple of years later, I had to go to the local police station and pay a fine of 1500 baht. Everyone told me I had 7 days to leave the country, but not true.