Multiple copies of a plugin can be adding, leading to fatal errors

[MC_incubus]

If you activate a plugin and doubleclick the "activate" link, it will activate two copies of the plugin, which leads to Fatal error: Cannot redeclare function_name()

plugins.php should make sure the plugin it is activating is not already activated!

[michel v]

Added a check to avoid activating the same plugin twice, and changed include() into include_once() to avoid including the same plugin file twice.