If - for any reason - the installation went wrong, chances are that some data have been written to the DB prior the failure. These data can prevent further installation, because Joomla believes the extensions is already there.
If Joomla is complaining about "conflicting menu items", you should inspect the #__menu table (using a proper DB client a la HeidiSQl or a web-bases one like PHPMyAdmin) to see if there are DB entries referring to Custom Properties (e.g. COM_CP_ABOUT, COM_CP_CONFIG, etc), and then delete them.
