This is by far the most requested feature.

People want an easy way to take multiple snapshots of websites over time.

Here's how archive.org does it

For people finding this issue via Google / incoming links, if you want a hacky solution to take a second snapshot of a site, you can add the link with a new hash and it will be treated as a new page and a new snapshot will be taken:

echo https://example.com/some/page.html#archivedate=2019-03-18 | archivebox add
# then to re-shapshot it on another day...
echo https://example.com/some/page.html#archivedate=2019-03-22 | archivebox add

Edit: as of v0.6 there is now a button in the UI to do this ^