The screen options you reference do not overlap, as they are designed to protect against two different types of attacks. Let me start with the syn-ack-ack proxy.
This is designed to protect a server from having its resources exhausted. Lets say an authentication user initiates a telnet connection to a server protected by the SRX, the user sends a SYN segment to the telnet server. The SRX intercepts(proxy between the user and the server) the SYN segment, creates an entry in its session table, and proxies a SYN-ACK segment to the user. The user then replies with an ACK segment. At this point, the initial three-way handshake is complete. The SRX then sends a login prompt to the user. A legit user will log in, however the attacker with malicious intent, does not log in, but instead continues initiating SYN-ACK-ACK sessions, the firewall session table can fill up to the point where the device begins rejecting legitimate connection requests. To prevent such an attack, you can enable the SYN-ACK-ACK PROXY Screen option. After the number of connections from the same IP address reaches the SYN-ACK-ACK proxy threshold, the SRX rejects further connection requests from that IP address. By default, the threshold is 512 connections from any single IP address. So a session is never established betweem the user and the server and the SRX holds the connection so only valid sessions can be established.