iRule to limit duplicate submissions to form

This may be over simplistic for what you're looking for, but I believe the basic functionality is there:

when RULE_INIT {
     user-defined: time to wait after failure
    set static::wait_time 30

     user-defines: max query attempts
    set static::max_attempts 3
}
when HTTP_REQUEST {
    if { [HTTP::method] equals "POST" } {
        HTTP::collect [HTTP::header Content-Length]
    }   
}
when HTTP_REQUEST_DATA {
    set user [URI::query ?[HTTP::payload] user]
    if { $user ne "" } {
        if { [table lookup -subtable USERQUERY $user] eq "" } {
            table add -subtable USERQUERY $user 1 $static::wait_time
        } elseif { [expr [table lookup -subtable USERQUERY $user] >= $static::max_attempts] } {
            HTTP::respond 200 content "Queries exceeded"
        } else { 
            table incr -subtable USERQUERY $user
        }
    }
}

If the request is a POST, collect the payload. If the user value is in this POST request, grab it. If there's no table entry for this user, create a table entry and give it an idle timeout value as specified by the "static::wait_time" value. Else if the table entry exists and its value is greater than or equal to the specified "static::max_attempts" value, respond to the client with an error message. Otherwise increment the value attributed to that user query in the table.