CentOS file owner changed on editing

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http://unix.stackexchange.com – I have the following problem. On Unix server (CentOS 5.5) a number of users belonging to the same group have access to the same files (via ssh). After one particular user (let's say usera A) has edited the file the file owner is changed to A, the group (G) and the permissions (0664) remain intact. Once a second user (B) tries to modify the same file he is not allowed to do so. If on other hand the user B modifies the file then A can change it without problems (except screwing up the permissions). I understand that there is something screwed up with the permissions/groups but I can't find out (HowTos)