Select the range (or entire column) to check for duplicates
Go to Home → Conditional Formatting → Highlight Cells Rules → Duplicate Values
Choose a formatting style and click OK
To find duplicates with a formula: in a new column enter `=COUNTIF($A$1:$A$100,A1)>1` (adjust range and cell)
Copy the formula down the column
Filter the new column for TRUE to show only duplicate rows
To return only duplicate values: use `=FILTER(A1:A100,COUNTIF(A1:A100,A1:A100)>1)` (adjust range)
To remove duplicates: select the range → Data → Remove Duplicates → choose the column(s) → OK
To count duplicates per value: use `=COUNTIF($A$1:$A$100,A1)` in a new column
To list duplicate rows uniquely: use a PivotTable with the column as Rows and Values set to Count, then filter counts greater than 1