Check subscription overlap when approving invoice

Overview:

When approving an invoice with subscription information the system will try and validate that a similar resource has not been paid for, for the subscription interval found in the invoice line(s).

Use cases:

Librarian is paying an invoice for a subscription that covers dates they have already purchased from another vendor.
Librarian is paying an invoice for a subscription term that they were already invoiced for and paid.
Librarian does not want the system to check for overlapping subscriptions because they are aware this is a duplicate purchase

Requirements:

• User can indicate for individual invoices whether sub dates should be checked for overlap
• User is alerted if possible overlap is found on other invoices. AND those invoices are shown to user
• "Overlap" is determined by matching vendor number, description, and dates within the subscription period?