APM iRule Event: insert Cookie after Logon Page

I noticed the earlier rule though inserted the cookie to application server in the first request, there were 2 cookie headers.

Cookie: F5_ST=1z1z1z1402414804z604800; LastMRH_Session=a66fdead;

I guess, application server picked the first cookie and couldn't find the one it was looking for.

Set-Cookie is the request to the browser to store the cookie.

Try the amended rule.

when HTTP_REQUEST {
  set flag 0
  if { [HTTP::cookie MyCookie] ne "" }{
     set flag 1
  }
}
when ACCESS_ACL_ALLOWED {
  if { $flag eq 0 }{
     HTTP::header replace "Cookie" "[HTTP::header "Cookie"]; MyCookie=MyValue"
  }
}
when HTTP_RESPONSE {
  if { $flag eq 0 }{
      HTTP::header insert "Set-Cookie"  "MyCookie=MyValue;path=/"
  }
  unset flag
}