Question

Using this to try and detect if a tweet with the same ID has already been added to the DB (and not add it if it has).

INSERT INTO `Tweets` (`TweetID`, `Time`, `UserName`, `UserNameID`,`RealName`, `ProfilePic`, `Tweet`)  
VALUES ('1234567890', 'Fri, 27 Jan 2012 00:29:39 +0000', 'user', '666', 'user', 'http://...' , 'tweet')
ON DUPLICATE KEY UPDATE `TweetID` = '1234567890';

but its still being added.

Answer 1

You need to create unique index that covers single TweetID field

CREATE UNIQUE INDEX uq_tweets_tweetid ON tweets (tweetid)

Personally I’d recommend you to remove ON DUPLICATE part and just use INSERT IGNORE INTO instead – as long as you don’t need anything to be modified in case of duplication