Asking MYSQLI to UPDATE a record if column A AND column B matches the input from the form, instead of creating a new record

Question

I have an HTML form that people will use to apply for vacancies in our company, and just one SQL table (applicants) that this form is writing into (with PMKEY appl_id (AUTO INREMENTED)).

I am trying to eliminate duplicate entries into the db from people reapplying for the same position over and over.

Each position has a vacancy code (vacancyCode), and I would like to check whether the applicants email address (appl_email) AND the vacancy code has been used together already, and then I would like to update that previous record, instead of creating a new one.

Could anyone please advise how to write such a statement?