Convert 2D table to 1D table
Make a pivot chart for the two-dimensional table, then remove the check of the row and column, double-click the cell of the summed value, and the corresponding one-dimensional table will be automatically generated.
converts to:
find duplicates
1: countif function 
2: Data--Sort and filter--Advanced, in the advanced filter pop-up box, check to select unique records.
3: Start - Conditional Formatting - Highlight Cell Rules - Repeat Values
4: Make a pivot table corresponding to the column data in the current worksheet
5: Data--Data Tools--Remove Duplicates
6: As shown in the figure: 
The formula of the second duplicate item is: =COUNTIF(A$50:A50,A50)
7: On the basis of 6, data - sorting and filtering - filtering, and then pick out items not equal to 1