Use hash values for TikZ externalization cache names.

tikz-external-hash.sty

% Copyright (c) 2019 Takahiro Ueda
%
% This file may be distributed and/or modified under the conditions of
% the LaTeX Project Public License (LPPL), either version 1.3c of this
% license or (at your option) any later version. The latest version of
% this license is in:
%
%    http://www.latex-project.org/lppl.txt
%
\ProvidesPackage{tikz-external-hash}[2019/10/01 v0.1.1]

% This style file replaces the \tikz command and the tikzpicture environment
% in such a way that externalization uses hash values of picture contents as
% the cache file names.
%
% Usage:
%   \usepackage{tikz}
%   \usetikzlibrary{external}
%   \usepackage{tikz-external-hash}%  before \tikzexternalize
%   \tikzexternalize
%
\usepackage{tikz}
\usetikzlibrary{external}
\usepackage{etoolbox}
\usepackage{environ}

% Patch \tikzexternal@tikzpicture@replacement in tikzexternalshared.code.tex.

\NewEnviron{tikz@external@hash@tikzpicture}[1][]{%
  % Here the cache file name is derived from
  % (1) the contents in the picture,
  % (2) the options,
  % (3) \beamer@pgfextension if defined.
  \edef\tikz@external@hash@tempa{#1}%
  \@ifundefined{beamer@pgfextension}{}{%
    \edef\tikz@external@hash@tempa{%
      \tikz@external@hash@tempa;\beamer@pgfextension}%
  }%
  \edef\tikz@external@hash@tempa{\expandonce\BODY;\tikz@external@hash@tempa}%
  \expandafter\tikzexternal@computemdfivesum\expandafter{%
    \tikz@external@hash@tempa}%
  \xdef\tikzexternal@nextfile{\pgfretval}%
  % Now, put the picture.
  \let\tikzpicture=\tikzexternal@tikzpicture@replacement@orig
  \begin{tikzpicture}[#1]%
    \BODY
  \end{tikzpicture}%
  \let\endtikzpicture=\relax
}

\let\tikzexternal@tikzpicture@replacement@orig=\tikzexternal@tikzpicture@replacement
\let\tikzexternal@tikzpicture@replacement=\tikz@external@hash@tikzpicture

% latest-raw-url: https://gist.githubusercontent.com/tueda/c9a9c073c5f656e7ce870faf2e836be8/raw/tikz-external-hash.sty