PHP Version 5.2.1

I've taken a look at my database and looks like it cuts everything after the diacritic letter like this: word "spo?eczno?ci" was cut to spo. Hell, how come?

An error was encountered
File: /home/deathplanter/www/forum/include/search_idx.php
Line: 149

PunBB reported: Unable to insert search index words

Database reported: Duplicate entry 'si' for key 1 (Errno: 1062)

Well, I was testing punbb to be my project's forum and when I made a "hello thread" in my mother tongue (yes, I used diacritic letters and what's weird, this doesn't happen when I don't), such an error appears after I've posted a message:

Error: Unable to insert search index words.

The funny thing's that at my mate's forum such thing does not happen. All the tables' encoding settings are set to utf8_general_ci and I don't know how to debug the script so I would see what's wrong. Could anyone try to help me?