Some formulae/casks contain duplicate checkable URLs or contain
URLs that become duplicates after `#preprocess_url` is used. With
the former, a formula's `stable` and `head` URLs can be the same if
they both use the Git repository. With the latter, a formula's
`homepage` and `stable` URLs are different but `#preprocess_url` can
return the same Git repository URL for each (which can also be
a duplicate of the `head` URL).
The `fabric-completion` formula is a worst case scenario, as it
contains both of these conditions but the repository has no tags.
By default, livecheck would needlessly check the repository three
times (i.e., no versions are found so livecheck tries the next URL,
which happens to be the same URL).
This commit avoids duplicate URLs in `#checkable_urls` and keeps
track of checked URLs in `#latest_version` to avoid a duplicate
caused by `#preprocess_url`. This should effectively resolve the
issue of checking the same URL more than once for a given
formula/cask. Checking the same URL only once across all the
formulae/casks in a given livecheck run will be handled in a later
commit (though I've done most of the groundwork already in previous
PRs).
bd4124c10f