If you see this error message on any of your invoices, it means the invoice number already exists in Xero.
You'll want to login to your Xero account and use search to find the invoice:
If it's an old invoice
Go to the new invoice back in Fergus and click Edit from the ⚙ cog menu.
Add (2), -1, or something else to the invoice number to differentiate it from the one in Xero and click Save.
Click the ⚙ cog menu again and click Send to Xero. The invoice should sync within 1 minute.
If it's the same invoice or search doesn't find anything
If the invoice in Xero is the same invoice that you're having trouble with in Fergus or search doesn't return any results, get in touch and we'll work out what's going on.