Voucher number is the only check point for the Tally when you are importing any data from XML to Tally.
In case your voucher numbering method is automatic than there is a high possibility that your data gets duplicated when the same XML or voucher has been imported in Tally.
To prevent duplication of vouchers keep your voucher numbering method as "Manual" with Prevent duplicate option as "Yes"
In case if you are trying to modify the numbering method of your existing voucher type than their is a possibility that your old numbers will get removed automatically and Tally does not allows to Enable the option "Prevent Duplicate".
So we are strongly recommend you to create a new voucher type and set voucher numbering method to manual with prevent duplication = Yes