Use of flowbits in Snort to alert upon reception of second identical packet

[stephane potier via Snort-sigs <snort-sigs () lists snort org>]

Hi,
I am trying to write a snort rule that alert only if the same packet is received twice (ip, port and content are 
identical). 
Idea is to raise the alert only in case of reception of the second same packet. Several other frames can be received 
between those 2 same packets.
I do not find clear indications how using flowbits for this case. I have found a rule that seems to do similar job in 
/etc/snort/rules/web-client.rules (see below), but I am not very clear how it really works. 
alert tcp $EXTERNAL_NET any -> $HTTP_SERVERS 443 (msg:"WEB-MISC SSLv2 Client_Hello with pad request"; 
flow:to_server,established; flowbits:isnotset,sslv2.client_hello.request; flowbits:isnotset,sslv3.client_hello.request; 
flowbits:isnotset,tlsv1.client_hello.request; byte_test:1,<,128,0; content:"|01|"; depth:1; offset:3; content:"|00 
02|"; depth:2; offset:6; flowbits:set,sslv2.client_hello.request; flowbits:noalert; classtype:protocol-command-decode; 
sid:2659; rev:4;)
Any explanation of the previous rule and idea is welcome. Particularly how flowbits isnotset and set can be written in 
the same rule, and their position in the rule.
Thanks.
Herl

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