@
bharath
You can use a formula based CF rule to highlight such items. To prevent duplicate rules from being entered, you can apply same logic in custom data validation too.
For CF:
Add a rule with this
=COUNTIFS($C$2:$C$21,$C2,$D$2:$D$21,$D2)+COUNTIFS($C$2:$C$21,$D2,$D$2:$D$21,$C2)>1
and apply formatting.
I leave the DV rule for your imagination.