When core mod_mam is combined with community mod_carbons on prosody 0.11, the Carbon-copied messages will not contain the <stanza-id> element required for MAM deduplication. This will lead to messages getting duplicated randomly, depending on message type, client priority, wind direction and moon phase. This is not an actual bug report (because Zash told me this is not a supported combination, and I've moved to core mod_carbons now), but merely for documentation purposes for the ones who come after me. But you might consider making community mod_carbons not load on prosody 0.10+.