How do I find a duplicate in SQL

I would suggest window functions:

SELECT ml.*
FROM (SELECT ml.*, COUNT(*) OVER (PARTITION BY County, City, Zip) as cnt
      FROM MoratoriumLocations ml
      WHERE MoratoriumID = 20
     ) ml
ORDER BY cnt DESC, County, City, Zip;

This will show the complete rows with duplicates, which can help you understand them better.

See Preben's answer for how to find dups.

To avoid dups altogether consider creating an unique index.

You coul use group by and having

SELECT  COUNTY, CITY, ZIP 
FROM MoratoriumLocations
WHERE MoratoriumID=20
GROUP BY COUNTY, CITY, ZIP
HAVING COUNT(1) >1
ORDER BY County

If you want to get the full row details you can use a sub query in combination with the group by and having statements

SELECT x.*
FROM MoratoriumLocations x
INNER JOIN( 
  SELECT  COUNTY, CITY, ZIP 
  FROM MoratoriumLocations
  WHERE MoratoriumID=20
  GROUP BY COUNTY, CITY, ZIP
  HAVING COUNT(1) >1
) dups ON dups.County = x.County
  AND dups.City = x.City
  AND dups.Zip = x.Zip