Retrieving multiple arguments for a single option using getopts in Bash

I need help with getopts.

I created a Bash script which looks like this when run:

$ foo.sh -i env -d directory -s subdirectory -f file

It works correctly when handling one argument from each flag. But when I invoke several arguments from each flag I am not sure how to pull the multiple variable information out of the variables in getopts.

while getopts ":i:d:s:f:" opt
   do
     case $opt in
        i ) initial=$OPTARG;;
        d ) dir=$OPTARG;;
        s ) sub=$OPTARG;;
        f ) files=$OPTARG;;

     esac
done

After grabbing the options I then want to build directory structures from the variables

foo.sh -i test -d directory -s subdirectory -s subdirectory2 -f file1 file2 file3

Then the directory structure would be

/test/directory/subdirectory/file1
/test/directory/subdirectory/file2
/test/directory/subdirectory/file3
/test/directory/subdirectory2/file1
/test/directory/subdirectory2/file2
/test/directory/subdirectory2/file3

Any ideas?