WARC only does deduping within a single WARC, I'm talking about deduping across millions of WARCs.
[edit] Should add a link, this is a pretty good overview, but you can also look at implementations such as the new zeno crawler.
https://support.archive-it.org/hc/en-us/articles/208001016-A...
I know of the CDX index files produced by some tools but don't know anything about the details/that they could be used to dedup across WARCs, I've only been referencing the WARC file specs via IIPC's old standards docs.