One of the questions on the application is whether a person has previously applied for a visa in Thailand.  if the previous application was under a different first name, with a different gender marker on the passport, what should the answer be? 
Answer summary
The consensus among commenters is clear: if you have previously applied for a visa in Thailand, regardless of changes in your name or gender identity, you should answer 'yes' to the application question. It's advised to be transparent about your visa history to avoid potential complications with immigration.
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Click comment and we will ask for your email, then OTP, then name.
Biff *******
If you have, then the answer should be yes. There’s no follow up question to that.
Facial recognition technology and fingerprint detectors are agnostic to names and gender identity, I'd recommend avoiding getting caught in a situation where you have to explain yourself to immigration. Be transperent.