The specifications for TLS 1.3 0-RTT mention the following threat which an attacker might realize:
Network attackers who take advantage of client retry behavior to arrange for the server to receive multiple copies of an application message. This threat already exists to some extent because clients that value robustness respond to network errors by attempting to retry requests. However, 0-RTT adds an additional dimension for any server system which does not maintain globally consistent server state. Specifically, if a server system has multiple zones where tickets from zone A will not be accepted in zone B, then an attacker can duplicate a ClientHello and early data intended for A to both A and B. At A, the data will be accepted in 0-RTT, but at B the server will reject 0-RTT data and instead force a full handshake. If the attacker blocks the ServerHello from A, then the client will complete the handshake with B and probably retry the request, leading to duplication on the server system as a whole.
My question is: Where is the attack? At the end of the day, the attacker could as well just have passed his copy of the ClientHello to Zone B and get the same outcome (a full handshake). What am I missing?