Use dashes for repeated Author names in agsm Style

Question

Looking for some help in creating a custom bibstyle with natbib.

Just started an MSc and planned to do my writing using TeX, as I did for my UG. The institute I am doing my MSc at, however, uses some custom version of the Harvard System. I have managed to recreate most of what they ask for by modifying a copy of the agsm.bst file.

I am simply struggling with the last part.

This is how it looks right now but to make it match what the institute wants, I need to have dashes where an author has been repeated. (Only the first entry is real, I simply repeat it for example.)

This is how they present it in the guide but they don't seem to use TeX and suggest writing in Word and using a third party tool for reference management (I am not fond of the idea).

Edit. MWE. This is the initial call for the package:

\usepackage[authoryear,round,sort]{natbib}
\setcitestyle{round,aysep={},notesep={: }}

This is a example cite and and list build

\citep[35-37]{tedre2015science2}
\bibliographystyle{agsmuhi}
\bibliography{sources}

This is how book looks:

FUNCTION {book}
{ output.bibitem
  list.label.output
  " \harvardyearleft " list.year * "\harvardyearright " * output.nonnull
  author empty$
    { editor "author and editor" item.check }
    { crossref missing$
    { "author and editor" editor either.or.check }
    'skip$
      if$
    }
  if$
  title.field field.used =
    { skip$ }
    { format.btitle "title" output.check }
  if$
  crossref missing$
    { format.bvolume output
      format.number.series output
      format.edition output
      format.publisher.address output
    }
    { format.book.crossref output.nonnull
      format.edition output
    }
  if$
  new.block
  note output
  fin.entry
  write.url
}

And this is the output function:

FUNCTION {init.state.consts}
{ #0 'before.all :=
  #1 'mid.sentence :=
  #2 'after.sentence :=
  #3 'after.block :=
}
STRINGS { s t f }
FUNCTION {output.nonnull}
{ 's :=
  output.state mid.sentence =
    { " " * write$ }
    { output.state after.block =
    { 
      newline$
      "\newblock " write$
    }
    { output.state before.all =
        'write$
        {"" * write$ }
      if$
    }
      if$
      mid.sentence 'output.state :=
    }
  if$
  s
}

There is also this dashify function which I don't know if it has anything to do with it, but the only place I see it called is in the function for formatting page numbers.

FUNCTION {n.dashify}
{ 't :=
  ""
    { t empty$ not }
    { t #1 #1 substring$ "-" =
    { t #1 #2 substring$ "--" = not
        { "--" *
          t #2 global.max$ substring$ 't :=
        }
        {   { t #1 #1 substring$ "-" = }
        { "-" *
          t #2 global.max$ substring$ 't :=
        }
          while$
        }
      if$
    }
    { t #1 #1 substring$ *
      t #2 global.max$ substring$ 't :=
    }
      if$
    }
  while$
}