That's definitely not a common thing, unless you're fielding a new team or division. There is a limit though, yeah? So it could be a deciding factor even when hiring multiple positions from the same pool of candidates.

Every thank you letter I've sent wasn't a thank you letter at all. I call back to specific things the interviewer said during our interview, and make it one last opportunity to pitch myself as the best candidate.

Thank you emails on their own are kind of pointless, but it's a crucial tool for applicant to address anything that they realize might have been missed or to clarify something they thought was important.

It's a perfect opportunity to offer thanks and further your case for the position, but it should be relevant to the interview.