How to quickly calculate the intersection of two columns of numbers in a table

Dear, as shown below. Assuming that columns A and B are original numbers, paste the following formula into C 1 of column C, press "Ctrl+Shift+ ENTER" to end the input, and drop down a few lines until blank appears.

Please adjust the reference range in the formula according to the actual data. For example, if column A has the line 10, it will be changed to $ a $1:$ a $10; If there are 7 rows in column B, change it to $B$ 1:$B$7:

= if error (small (if (countif ($ a $1:$ a $6, $B$ 1:$B$4)>0, $B$ 1:$B$4), the first (a/kloc)