There are certain fields that you never want to contain duplicate values. This might be a field containing Employee ID or Invoice Number. You can easily protect a field from duplicate values by making it "unique" In other words, each value in the field can never match another value. Each one must be unique.
To make a field unique:
Within the Unique section, turn on the Require unique values for all entries in this field checkbox.
Click Save.
Note: You can't make a multi-value user field unique.