Not counting the intended double dose of the 2-dose vaccines such as Moderna and Pfizer.
I can imagine this in several instances. Say someone threw away their vaccine card after the second shot, and later learn they need that card to document their vaccination for an employer or school, and getting vaccinated again is the most expedient way to solve that. Or, they aim to do international travel, but they don't think their vaccine is on the list of that country's accepted vaccines, so they get a vaccine that is.
Are there known risks to doubling up on vaccines generally?
Is anything documented about the various COVID vaccines that warns against taking them when already vaccinated?
There is an existing question but it was confused for a "what if you split doses 1 Pfizer 1 Moderna" question.