Vague question, but if you are talking about a database field, the simple way is to create a unique index on the field (if the database supports it). The index will not allow duplicate values to be added to the database for that field and will generate an error if somebody tries.
give 2 ways of preventing damage to diskettes
five ways for green computing
Write a report on ways of preventing female forticide and making nation women friendly?
aasdasd
Use oil
Vervain such a pretty flower.
don't litter!
i dont no
painting and galvanizing
you can use the use of internet
People should not be throwing trash on the beach
Eat lots of candies and cake...