iRule to Remove Duplicate Header by Value

Via iRule, trying to remove duplicate Strict-Transport-Security headers (developers are unable to), the below code does not seem to work.

when HTTP_REQUEST {
 
  foreach a_header [HTTP::header "Strict-Transport-Security"] {
 
   if {[HTTP::header $a_header] eq "max-age=2592000"}{
 
     HTTP::header remove $a_header
   }
  }
}

The below seems to work but gets rid of Strict-Transport-Security as a whole, which I do not want to do:

when HTTP_RESPONSE {
 foreach header {Strict-Transport-Security} {
  HTTP::header remove "Strict-Transport-Security"
  HTTP::header remove "max-age=2592000"
  } 
}