Having a contact number listed only once is recommended to avoid confusion and ensure the best user experience. No database issues will occur if the same number is listed with two different contact types but this may be confusing to an end user viewing the record.
If a record has Home (800-555-4444) and Cell (800-555-4444) and no other phone numbers listed in either The Education Edge or The Raiser's Edge.
If a record has those same two contact types with the same number, but also has a third number in The Raiser's Edge, Cell (444-555-5544), end users may be confused by seeing two Cell numbers and unable to determine which number is appropriate for their task. In Integration, this will also cause an error in the Integration Activity Report that duplicate contact types cannot be added to The Education Edge.