it is because you already defined the 'abuse_id' as auto increment, then there is no need to insert its value. it will be inserted automatically. the error comes because you are inserting 1 many times that is duplication of data. the primary key should be unique. should not be repeated.
the thing you have to do is to change your insertion query as below
INSERT INTO `abuses` ( `user_id` , `abuser_username` , `comment` , `reg_date` , `auction_id` )
VALUES ( 100020, 'artictundra', 'I placed a bid for it more than an hour ago. It is still active. I thought I was supposed to get an email after 15 minutes.', 1338052850, 108625 ) ;