Prevent and locate duplicate worksheet entries

- [Instructor] Duplicate values, … particularly the unintended ones, … sometimes cause big problems … when you're working with data in Excel. … Here's a list on this worksheet that we're about to set up. … Eventually it's going to be quite large. … We want to make sure that when an ID number … goes into column D here … that it's unique, it's different from all the others. … No duplicates allowed. … There's no quick, easy way to do this, … but we can use a feature called data validation. … And we'll be using a function called countif. … Over in the column H, I've got a list of some regions here, … and a simple example of countif in cell G2. … It's simply in effect saying, looking at this range here, … how often do we see West? … Well, it's four times. … Now, selecting column D, … this is where the ID numbers will go, … let's go on the data tab up in the ribbon, … off to data validation. … You'll find it under data tools. … The icons might look a little different on your screen. … Choose data validation. …