Match Numbering | product class

All Items are present 1 or more times. Generally 2 times each (may vary). A sequence number is given to every item present. Sequence numbers of matching items have to be written.
Example: There are say 40 different items. Each item is present 2 times making it 80 items. These items are jumbled. Numbering of items is done from 1 to 80. Now, assume that item at Sequence Number 1 is matching with item at Sequence Number 23. So, answer would be 1,23.