Sorry to hear you're having installation woes. It's always tricky given the variance in how webfarms set up their servers.
So, I see a bunch of advice, including;
http://www.silverstripe.com/site-builders-forum/flat/1733
http://www.silverstripe.com/site-builders-forum/flat/2837
Which point to
(a) Increasing memory limit beyond 32MB
(b) Manually setting the .htaccess file's rewrite rule;
(-)RewriteRule .* sapphire/main.php?url=%1&%{QUERY_STRING} [L]
(+)RewriteRule .* /<<subfolder>>/sapphire/main.php?url=%1&%{QUERY_STRING} [L]
c) Noting that WebDrive uses Mod Vhost Alias:
http://httpd.apache.org/docs/1.3/mod/mod_alias.html
VirtualDocumentRoot /home/httpd/vhlinks/%0/
"I assume the VirtualDocumentRoot is causing the problem for SilverStripe." says a poster.
(d) A developer here sorted the issue by changing the core/ManifestBuilder.php line 39
// Config manifest
// $baseDir = dirname($_SERVER['SCRIPT_FILENAME']) . "/..";
$baseDir = '/path/to/files';
If you do get it solved (item d could be your savior) we would love to know precisely what you needed to do so that we can patch the software with a fix. Until someone does this, others will stub their toe on webdrive which is a shame.
Finally, do note that "cannot redeclare" implies it is including the same file or code twice, so I'd hope you didn't accidently unzip SilverStripe twice and that its somehow embedded in itself.