Approvals Work Area shows already approved tasks

Some of our users see Tasks in the Approval Work Area that are tagged 'Approval' with the option to approve or reject. These are old tasks that were already approved, we confirmed that on the Transaction Console. When the user tries to approve, an error message will display 'Couldn't Approve Task'. How to remove these from the Task list? When we tried to run the index 'Create Approvals Search Index' in a lower instance, it resulted in the error 'Index fnd-worklist-approvals-zdt already exists'.