Server migration & Let's Encrypt


I move all my sites to a bigger vps (the 2 vps are with Debian 8 & Virtualmin).
How do I need to do for the Let’s Encvrypt ssl ?
Revoke from the 1st server and how to ?

Here’s the beginning of the error that I get on new server when I ask for Let’s Encrypt ssl:
Parsing account key…
Parsing CSR…
Registering account…
Already registered!

Thanks -