It is a common practice for Applicants to use a Reference across multiple Opportunities, as the reference letter itself is often specific to the Applicant rather than the Opportunity. You may be concerned that this will cause a Reference to receive separate reference request emails for each Opportunity for which they are being used as a Reference.
If the Applicant has selected the Reference from the dropdown box at the reference request question in the application, the Reference will not receive separate reference request emails for each Opportunity for which they are being used as a Reference.
If the Applicant has entered the Reference separately each time via the "Add a new Reference" link, regardless of whether the Reference was already listed in the dropdown box, the Reference will receive a separate reference request email each time this has been done. This option is provided in those instances when the Applicant may want a completely different reference letter from the same Reference provider.