That would copy the user class, and therefore you would run into a number of issues, espcially if your two objects where being updated and then persisteted to the database you run into issues.
To me it seems like the issue is because you are running an old version of PHP (i.e. < v4.1). I had this on older versions of PHP. Maybe it would be an idea to confirm this by searching the PHP bug DB for this on older versions of PHP.
if you have PHP installed on Linux, the best way to upgrade is to call phpinfo() before you build a new version. From the result of that, copy the ./configure params and use that to build the new version.
It's a good idea to stop httpd when you do the make install bit.... and then start it after. If it doesn't update, do a locate libphp4.so and make sure the newly compiled module is copied over to the location of the active libphp4.so file (or you can tell it in the compile phase where the location of the current libphp4.so file is so it overwrites it)