If you want to add a new leg with already existing 'Quote number', Leon will issue a warning with the schedule information of the existing quotation.
On the screenshot below there is a trip with the 'Quote no.' 5544332211.
Below is an example of the warning that Leon issues - a new flight that is about to be added has the same 'Quote no.' as the trip above.
It is still possible to add such a flight - the warning is just an information.