Explanation:

Step 1 - Select the required fields and count
We want to group by CustomerAltKey and count occurrences.
SELECT CustomerAltKey, COUNT ( * )
Step 2 - From the table
FROM DimCustomer
Step 3 - Grouping by email
GROUP BY CustomerAltKey
Step 4 - Filtering duplicates (only values with count > 1)
HAVING COUNT ( * ) > 1
Step 5 - Limiting the result to 1,000 rows
LIMIT 1000
(or in T-SQL, SELECT TOP(1000) ... ). Since both options are shown, we use LIMIT 1000 because Fabric Warehouse supports Synapse SQL T-SQL + ANSI SQL style.
Correct Sequence:
SELECT CustomerAltKey, COUNT(*)
FROM DimCustomer
GROUP BY CustomerAltKey
HAVING COUNT(*) > 1
LIMIT 1000