Avoid playing both unavail and vm-intro on mailboxes?

I have a small asterisk installation, and some users don’t have a custom voicemail recording and just rely on vm-intro playing as a default. I have a user who has recorded a custom greeting to unavail.wav, and for that user, I am seeing that unavail.slin and vm-intro.gsm both play.

Is there a way to disable vm-intro.gsm only when unavail.slin is not present?