To make it clear. Which one is used / necessary?
Drupal Core
Or
JS Aggragator http://drupal.org/project/javascript_aggregator
If the module is needed you should check if it enabled.
If you use the JS Core Aggregator you shoul check, that there is not the JS module enabled.
Comments
Comment #1
mehmeta commentedDrupal core js aggregator has to be enabled for this module to work. It is checking whether it is enabled and warning the user if it's not, however the module is not checking whether other js aggregation related contrib modules are active at the moment.
Comment #2
bennos commentedCheck of other js aggregation modules are enabled and show a status message.
Comment #3
mehmeta commentedChecking whether javascript_aggregator is enabled and warning the user on the admin page now.