How to avoid Duplicates entry in multi columns in a table

one way use conditional formatting.
Another way by "Data Validation" :

If you want to prevent duplicate data from being entered into a range, here is a pretty simple solution by using Data Validation.
Select the range of cells in which you want to prevent duplicates (for example, A1:E12).
From the Data menu select Validation.
Click on the Settings tab.
From the Allow dropdown box select Custom.
In the formula box type =COUNTIF($A$1:$E$12,A1)=1
Click on the Error Alert tab.
In the Title box type "Duplicate Entry!"
In the Error Message box type "You cannot enter a value already in the list."
(Please refer attach file)