Error while enabling a core module I need to enable the 'Aggregator' module on my Drupal 7 website. When I do that, I get a error message saying:


  DatabaseSchemaObjectExistsException: Table aggregator_category already exists. in DatabaseSchema


I researched on this and what it means is that the table for the module already exists in the DB. Now in case of contributed modules I can disable them, uninstall it and make a fresh install and re-enable. I am not sure what can be or should be done in case of core modules. 

Any guidance is greatly appreciated.
You somehow have aggregator uninstalled with its tables existing, which should not be. By examining aggregator_schema() in aggregator.install you can see that upon installation aggregator will create several tables named aggregator_(something). Table names correspond to the keys of the array returned by aggregator_schema(). If any of those tables exist you must delete them from the database before attempting to enable the aggregator module. As always, back up and understand how to restore your database first before attempting something like this and rehearse on a test system.