Dashboard problems in 3.5

I have been trying to upgrade one of my sites from 3.4.2, which has been running fine, to 3.5+. I began by trying to leap straight to 3.5.1, but I encountered a series of problems in the admin interface; various pages, including the dashboard and the plugins page, would fail midway through loading and leave an error something like:

Fatal error: require(): Cannot redeclare class wp_comments_list_table in [path]/wp-admin/includes/list-table.php on line 42

I backed back down to 3.4.2, and tried updating to 3.5 to see if the problem exists there. It does.

This site has suffered for a while from the “Call to undefined function” problem described here — http://wordpress.org/support/topic/fatal-error-call-to-undefined-function-wp_dashboard_setup?replies=3 — which is solved by replacing:

require_once(ABSPATH . 'wp-admin/includes/dashboard.php');

in wp-admin/index.php and wp-admin/network/index.php with:

require_once('includes/dashboard.php');

And given that line 42 of wp-admin/includes/list-table.php contains the following:

require_once( ABSPATH . 'wp-admin/includes/class-wp-' . $required . '-list-table.php' );

I thought, hey, it’s the same problem. So I replaced it with:

require_once( 'class-wp-' . $required . '-list-table.php' );

Which works exactly once — the first time I reload the page after making the change, the full page loads, but after that if I reload again, or if I go to another one of the problematic pages I get the same error again. I’ve cleared caches, I’ve deactivated all my plugins, and I’m just totally at a loss.

I’ve searched around and can find nothing like this, so I’m really hoping somebody here might have an idea about what could be causing this. Many, many thanks!