The DNS GUI will not let you create two A records with the same name.
But, even though there is no notion of case sensitivity with DNS hostnames, you can actually get around the restriction in the GUI by creating the A records as:-
or if you needed to create 3 A records:-
Any subtle change in case is enough to fool the validation check in the GUI. When the entries have been saved they will all resolve as "server1" giving you the round-robin resolution I assume you are trying to achieve.
Hope that helps.