Eunice here from Shopify, thanks for posting!
Some orders can have the same order number, however, the order ID will always remain unique. The cause of this can be a high order volume or orders queued due to flash sales. If you depend on the order number for an API system you are using, then it's always recommended to use the order ID instead.
There is currently no workaround, however, we'd love to record your experience on this. Please visit the Shopify Help Center and log in to your account to create a support request.