Avoid duplicate items

[LuisData]

Hi, I need some help.

I have 2 list called A and B. 

List A has information as Name and rate. 

List B has a vlookup from List A with name. So, the user can select a name

I don´t want duplicates, for example: I create an Item with Luis M in list B, if I create a New Item Luis M is available. How can I avoid this? I want that a New Item doesn't available with existing items

[#2]

@LuisData The easiest way to do this is go into the List settings on List B, open the column that should be unique and select Enforce unique values.   Check it out and let me know if this works.