[TYPO3-english] Fatal error on upgrading from 6.1.7 to 6.2.0beta5

[Sergio Catalá]

Hi list,

I tried to upgrade from 6.1.7 to 6.2.0beta5 and removed the symlink 
typo3_src that points now to the new core.
However, when trying to get the Install Tool, I receive the next error:

Fatal error: require_once() [function.require-once]: Cannot redeclare 
class typo3\cms\about\package in 
/var/www/server/vhosts/host/htdocs/typo3conf/trunk/050_core/typo3_src-6.2.0beta5/typo3/sysext/core/Classes/Package/PackageFactory.php 
on line 53

Any idea how to solve this?
Any steps I missed?

Regards,
Sergio