getopt with several `--`

I have a script with this usage:

myscript [options] positional-args... -- [fwd-params]

Because [options] can have long, or short variants, I like using getopt. But I'm having troubles. I use getopt like this:

args=$(getopt -o a:,b --long alpha:,gamma -- "$@")
eval set -- "$args"
while : ; do 
  case "$1" in
    -a | --alpha) a="$2" ; shift 2 ;; 
    -b          ) b=true ; shift   ;;
    --gamma     ) g=true ; shift   ;;
    -- )          shift  ; break   ;; 
  esac
done

positionals=()
while [ $# -gt 0 ] ; do
  case "$1" in
    *  ) positionals+=("$1"); shift ;;
    -- ) shift ; break ;;
  esac
done

# What-ever is left in "$@" needs to be forwarded to another program

backend "$@"

This works great if I don't have any [fwd-params]:

$ getopt -o a:,b -- -a 1 -b pos1 pos2
 -a '1' -b -- 'pos1' 'pos2'
            ^-- getopt adds this to help me find 
                the end-of-options/start-of-positionals

But it falls apart if the user defined any [fwd-params]. Here's my desired output:

$ getopt -o a:,b -- -a 1 -b pos1 pos2 -- fwd1
 -a '1' -b -- 'pos1' 'pos2' '--' 'fwd1'
                              ^
                              \-   I'll use this to delimit 
                                   the positional arguments 
                                   from the forwarding ones.

And here's what I actually get. The user's intentional -- has been filtered out.

$ getopt -o a:,b -- -a 1 -b pos1 pos2 -- fwd1
 -a '1' -b -- 'pos1' 'pos2' 'fwd1'

What's the best way to delimit my positional-arguments from the forwarding ones?