If I have two cpanel machines that are clustered to share DNS information, what happens when I create and account on Server2 that matches an existing account on server1? Will the DNS still keep the site at server1 or will the DNS try to flop it over to server2? Or will it just be confused?

the older zone record with the same domain name would be over written with the data from the newer record.

is there any solve of that problem instead of checking each time: is already that lgoin name on the second server ?

you can have same login (user) names on both the servers without dns cluster problems, just not same domain name. There's no cpanel feature I'm aware of that would check other servers in a cluster for duplicates before allowing you to add a new domain.

Aha... so what you are saying S3 is that because I can have the same logon names I can have the same plan/package names - just not the same domain name. This helps as I am installing Modernbill and it makes life easier if I have the same plan names on both servers.

I screwed it up last night - used the same domain name.

you got it.