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] or [yyyy][mm][dd]

Recommend Pattern:

- `ORD-[mm][dd][yyy][3counter]`; Counter step: `1`; Counter by Store: `No`; Reset Counter: No
- If your Magento store has multistores, try this: `ORD-[store_id][mm][dd][yyy][3counter]`; Counter step: `1`; Counter by Store: `Yes`; Reset Counter: No

If you still cannot resolve the issue, you can get help here

Published on: 06 / 13 / 2019