Access has a query wizard to find duplicate values in a table.
First, add an autoincrement column to the table like others suggested.
Second, run the Find Duplicates query wizard to generate a query that finds all the duplicate rows.

Use that  Find Duplicates query as the basis to generate and fine tune a  Delete query that gets rid of the duplicate rows by eliminating the rows having an autoincrement value higher the lowest one found for each duplicate.

Then delete the autoincrement column from the table and then re-add it again. This should work as long as you haven't used the column or table in a Relationship.

Now you should have consecutively numbered rows with clean nonduplicated data in a table that generates it's unique row autocount ID automatically.