Secondary Server - how much validation

We have a validated 3-tier system (Client, Application server and DB).

Now we are planning to introduce a secondary Application server in the system. This secondary Application server will be identical to the primary server.
Note: We will be using a load balancer which will include the two Application servers.

Now the question is how much validation is needed to validate the secondary server in the system? I think the full validation that was done for the system with the primary server is not needed. Any thoughts?

Thank you,

If you can prove that the new server is identical to the current server then there is a case for this (Like for like). Justification should be straight forward when you do this.

We also have our own validation server template that may come in useful.