Search for Duplicate IDs

a. Navigate to Reports > Saved Searches > All Saved Searches > New.
b. Choose any type of search
c. On the Results tab:
    i.  Choose the field value where we would screen duplicate entries among the list (e.g. Name)
    ii. Set Summary Type to Group
    iii. set Internal ID and set summary type to Count.
d. Rename and save the search.

Notes:
The Internal ID serves as a unique data to differentiate each record in NetSuite.
The Duplicate Detection (Lists > Mass Update > Mass Duplicate Record Merge) only show similar records where the first few characters of data are matched (Setup > Company > Duplicate Detection) and screened.
Saved Search would only show number of duplicates. Also it would not work if you drill down to its results.