Order ID has been conflicted
You may setup custom order number patter too simple, this cause of conflicted Order, Invoice, Shipment, Credit memo ID issue.
Problems
Problems may be known:
Missing Counter
The counter was reset to beginning and there is no [dd], [mm], [yyyy] added.
Solution
Make sure create an unique incremental ID.
Add counter number: Recommend >3= e.g: [3counter]
Add purchased date: [mm][dd][yyyy], [dd][mm][yyyy] orSome readersCustom Order Number User Guide
Custom Order Number
Overview
With the ID of Order, Invoice, Shipment or Credit Memo, the default ID of Magento only includes the numbers and the ID of the first order is 000000001. Therefore, with newly established websites, the order quantity is low, which makes customers feel that this store has no the high reliability. This leads to the low number of purchasing quantity of customers at your store. Besides that, to manage the orders of customer that are created on whichever date, month,Few readers