You may have duplicate review tasks for an entry for a few reasons. For instance, a review stage was initiated more than once or you have some test review tasks that are no longer needed. You can easily delete review tasks so the Manage tasks list is an accurate representation of completed and pending review tasks.
Delete a review task
Open the Manage workspace and go to Review flow > Manage tasks
Click on the checkbox next to the review task you want to delete
Open the Action drop-down and choose Delete
Undelete a review task
In the Manage workspace, go to Review flow > Manage tasks
Open the All but deleted drop-down and toggle to Deleted
Select the checkbox next to the review task you want to undelete