User is trying to submit time entries, but cannot do so and gets a message that the entries are already submitted.
This can happen if the time entries are already billed and approved. You can easily identify such entries as the time entry rows are color-coded. They display a colored bar on the left: red indicating a non-billable entry, green indicating a billed entry and gray indicating entries locked for billing. You can also see the Submit or Approve status column in the grid list. Check Core help for details.