Insert record into table (if record does not already exist)

Hello,

How do I insert a record into a column, if that record does not already exist.
If it does exist, then do nothing:

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INSERT INTO _table (_item) VALUES ('001') ..... ?

If _item 001 does not exist, then insert it, else do nothing.
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Please note that _item IS NOT a primary key, but I can't have duplicates there.

I'm using mysql 5.1.


Thanks,
Rick