When highlightElement is called on already-highlighted code (e.g., consecutive hljs.highlightAll() calls), the existing <span class="hljs-*"> tags from the previous highlighting are incorrectly flagged as "unescaped HTML" - a false positive security warning.
This fix checks for the data-highlighted attribute early and silently skips re-highlighting, matching the proposed solution in issue #3761.
The fix prevents the false warning when calling hljs.highlightAll() multiple times on the same code blocks.
Fixes #3761