l need a user to enter a start and finish number, then l need to check that
there are no duplicate numbers from previously entered start and finish
numbers, in another preadsheet.

ie

entered 10,000 to 10,500

Check entries
1 to 1,000
6,000 to 9,000
11,000 to 11,500
10,200 to 10,700

Duplicates found 10,200 to 10,500