Calendar Bug: Duplicate entry 'rowing-15156' for key 1

[Ibiza MF]

SMF 1.1. RC2

Koodi [Valitse] Laajenna

Duplicate entry 'rowing-15156' for key 1
File: ......./forum/Sources/Subs.php
Line: 451

I have attempted to change event date in calendar, and this error was created.
Later same error appears If I try to delete event.
I have deleted it's topic from forum and manually deleted event from 'smf_calendar' table. And even then if I try to add new event it displays same error!