Find a set of re-occurring values in more than one columns

Can anyone help me in finding a way where I could quickly track down all the instances where column-A and column-B have a set of re-occurring more than once in the sheet.
When the re-occurring values are found, the suggesting formula/function/trick should be able to do either of the following actions:

Highlight re-occurring values in both Column-A and Column-B
Write a word "Duplicate" in next column [Column E or F]

Excel sheet is attached for reference

Yes but your question is unclear, please post what the desired outcome is?

it may be if( countifs(c:C,c2,D:D,d2)>1,"Duplicate","")
or if( countifs(c$1:C2,c2,D$1:D2,d2)>1,"Duplicate","")

Or

Formula:

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Thanks davsth,
Your formula perfectly works for me. Stay blessed!