Hi Will,
What we mean by "not being supported" in an active-active configuration is that if a takeover occurs and dedupe is running, dedupe will stop. I don't think this is a big deal, it just means that you need to restart it manually after giveback, or just wait until the next scheduled dedupe process kicks in. Here is how I described it in the Dedupe FAQ:
27. Will deduplication work in a clustered (CFO) environment?
Yes, the deduplication operation will suspend during takeover and can be resumed after giveback. During the takeover period, deduplication will continue to create and log digital fingerprints as new writes take place on the volume. Note: deduplication must be licensed on all CFO cluster nodes.
Does that help?
DrDedupe