In excel sheet, if we want to compare the values in column A vs column B, what is the best formula or function used to identify duplicated values?

The best and easiest way formula to Identify duplicates in the excel are using Conditional Formatting > Highlight Cells Rules > Duplicate Values.

You can Try to use also VlookUP.

You can use Conditional Formatting and then click "Highlight cell rules"  and then "Duplicate values". It will make duplicate values easily apparent.

You may also try Alt + A + M. This will remove the duplicate values in a line.

You may use a conditional formula to get the duplicates, here is a commonly used syntax;

'=IF(ISERROR(MATCH(A1,$C$1:$C$5,0)),"",A1)'

or use VLOOKUP