The Outlet Tonnage report will, by default, list every non-deleted ticket that has weights in your system.
Where the weighbridge ticket relates to another job (e.g., Skip, material), the ticket is stored against the internal weighbridge account. This can lead to the report seemingly duplicating as the weights will be recorded against the main ticket and the weighbridge.
These internal tickets are typically marked not set for invoice creation and are essentially for reporting purposes only.
If you are seeing duplicate weights on the report, go back to the filters and ensure that you have selected "External" for the Account Type filter. This ensures that tickets against the internal account are not used for the report.