strange behaviour of auto_increment primary key

[Czeslaw Liebert]

i have a table with a primary auto_increment key field of integer type.
this table has been running error-free for months and suddenly it stopped to do so a week ago. when i add a row to it, it sometimes adds the row three or four times, each time with the same ID!!! i get one row repeated three times with the same primary key!
when i try to delete one of them via my php scipt, which deletes rows with WHERE id='nn' clause then all three/four rows are deleted; when i try to delete it with phpMyAdmin then they r deleted one by one.

what could be the cause of such strange behaviour?

the mysql ver is: mysql Ver 12.22 Distrib 4.0.18 running on openBSD-3.5