how can avoid that a session writes in other session file?

[zurron]

I'm using sessions, and till now all was working fine, but now without any logical explanation when I open a new session this session is store in the file of a previous active session

so if a user log-in I have a file like this
less sess_6a2d99e67c4d8dcc6056df29c7cf0074
user_name|s:4:"0001";

and if a second user logs-in the content of this file is changed for the new data of the second user

less sess_6a2d99e67c4d8dcc6056df29c7cf0074
user_name|s:4:"0002";

the funny thing is that only happens with the 2 first users that logs-in with the next users it works fine.

If it has happened to someone and knows how fix it, it would be nice a little help

thanks

zurron