If you see the warning message "Warning - Orders in report period may be duplicated" in the Order History, it is due to an order attribute spanning two different dates.
There are three different ways to view report data.
Based on Order History Report (default) - This records the sale / payment data on any day that the order was created / reopened / closed on.
Based on Order Create Date - This records sale / payment data on the date of the order that it was created on.
Based on Transaction Date - This records sale / payment data on the date of the order that funds transfered between the cashier and the customer