Notation for a unison double stop

Question

What's the standard notation for the occurrence of the same pitch played on two different strings of the same instrument simultaneously, to be used within the same staff?

The typical context would be a double stop lick that needs to resolve in unison, perhaps on the tonic. The reason for playing the same note on two strings rather than one might be uniform volume across the lick or just the special quality of the sound. It might be quite inconvenient to split the entire double stop lick into two separate staves just to avoid the local issue of notation.

I understand that I could just write "On D and G" above a G note (if using English), but I wonder whether there's any established culture neutral notation as an alternative to that. I don't care whether the individual strings are notated explicitly, or just the "doublestopness".

Answer 1

I would agree with @user1079505 about using two noteheads.
Elaine Gould, 2011, Behind Bars, p.397: p.398: p.399:

Answer 2

For notes with a stem one way would be for the note to have a double stem, one pointing up and one down.

For a whole note (no stem) this isn't possible so the next best way is to indicate the fingering with "0" above and "4" below the note indicating that the note should be played with both the 4th finger and open string.

Here is an example showing both used at the same time for the half note at the end of the third bar -

Answer 3

The question's been answered but the same convention of note stems going in different directions is also used when the notes to be played with the same pitch on different strings aren't played as a double stop but are instead played sequentially. Here's an example from the last page of Bach's Chaconne in D minor:

(James Ehnes plays this passage here)

and here's an example from the first page of the prelude from Bach's partita in E major for solo violin

with Midori playing it here.