No, both will be allowed. Without port overloading there is 64k possible source NAT translations on single interface. With port overloading you can have up to 32x64k translations as long as there are different destination addresses.
Source NAT doesn't only change SRC IP but also (by default) SRC PORT. So when two local hosts want to connect to the same DST IP & PORT firewall will translete them to different SRC PORT and this way will be able to differentiate them.
126.96.36.199:9999 -> 188.8.131.52:80
184.108.40.206:8888 -> 220.127.116.11:80
18.104.22.168:6666 -> 22.214.171.124:80
126.96.36.199:7777 -> 188.8.131.52:80
With port overloading you can have multiple (up to 32) translations from same SRC PORT when sessions are destined to different DSC IP.
To expand on what wuddy and alex said, this is a special scenario that you turn off port-overloading.
By default, a single IP address can allow 65,535 ports with about 64K unassigned ports available for use.
Port-overloading is enabled by default on the SRX which will increase this number based on the following: This feature can potentially allow the same IP/port combination to be used by the same host when opening multiple sessions concurrently but to different addresses. For e.g:
session 2 = 10.10.10.10:4444 -> 184.108.40.206:80 <===This is theorectically possible!
In order to allow external SIP clients to connect to Internal SIP phones, the STUN client/server communication requires that the IP/Port be the same and consistent. This could break the end to end communication with STUN clients. When configuring the any-remote-host persistent NAT type with Interface NAT, you have to explicitly disable port overloading.
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