Unable to add new technician eventhough the email already been clear from other instance

Unable to add new technician eventhough the email already been clear from other instance

I'm using cloud SDP, currently I'm trying to add 1 new technician, prior before this this technician email had been used on another instance. 
But the email currently already been deleted from the other instance. It's been overnight & yet when I'm trying to add this email to my current instance it still prompt : "Email entered is already in use. Please enter different email."

What is my solution to this?

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