Hi @Tanry,
In this case you will have hit the Let's Encrypt duplicate certificate rate limit. If you don't have other names to include on your certificate (which could potentially make it non-duplicative with the earlier certificates), you'll have to wait a week before you can issue more Let's Encrypt certificates again. You could also try using a different ACME CA as there are a couple of others out there that may give you a free certificate using the same client software (with different policies than Let's Encrypt's).