"I'd like to ask for help understanding practices or conventions or ... among Portuguese Jews".

These naming conventions are different in different locations. I have never seen a child in Amsterdam with a double given name, with the parent's name the second half. Western Ashkenazi have the child's given name then the father's, without the 'ben' in between; this might look like a double name, but it isn't: Marum Moses is Marum the son of Moses.

The surname confusion was mainly early on, after the Sephardi became calmer about the Inquisition, they gave one surname to all their kids, and, conveniently, they differentiated different families, with cousins all named the same (9 Duarte Daiz named after the first) by appending the mother's surname, like many Spaniards still do today, but a permanent addition, leading to Vas Nunes da Costa de la Penha as one surname.

But I know that Joseph da Costa was Joao Perez da Cunha, so it isn't a problem in the records. The problems occur when many use the same aliases, so you don't know which person it is. I always assumed that was to confuse the Inquisition, as many traveled to Spain/Portugal for business.

Sally Bruckheimer
Princeton, NJ