Doubts: email, restriction, note, interview email

Hi.
If you can help me, please.

1) How can I restrict the registration of only one candidate per job vacancy?
2) The company receives the email stating that the registration was successful, but the candidate is not receiving the email. What can it be?
3) When I open the details of the candidate, his data appears with an option to include a note for him, but when I click on it, nothing appears for me to be able to post it.
4) When I register an interview, is it possible for the candidate to automatically receive an email informing them of the date and time of the interview?

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