Everyone has a spelling for their first name that is the one they use. It may be the spelling given at birh or one they adapted later, and that spelling is definitive to them.
But as far as names go as a generality, there are no definitive spellings, There are common and traditional spellings - and then the variations.
In my DGD's class at school, at one time, there was an Isobel, Isabel and Isabelle. Same name, different spellings, the first one is usually considered to be the traditional Scottish way of spelling the names, all equally valid.
So (to link back to the thread) Whether the Duchess of Sussex spells her name 'Megan', 'Meghan' or any other version, is a matter of her personal choice and cannot be right or wrong for any reason.