@achow101 noted we don't reject them therefore they must be silently accepted. This is not part of the specs, we should remove them. Also cc @sipa.

I can take care of this.
{
    const auto& it = m_last_hardened_xpubs.find(key_exp_pos);
    if (it == m_last_hardened_xpubs.end()) return false;
    xpub = it->second;
    return true;
}
DescriptorCache DescriptorCache::MergeAndDiff(const DescriptorCache& other)
{
    DescriptorCache diff;
    for (const auto& parent_xpub_pair : other.GetCachedParentExtPubKeys()) {
        CExtPubKey xpub;
        if (GetCachedParentExtPubKey(parent_xpub_pair.first, xpub)) {
            if (xpub != parent_xpub_pair.second) {
                throw std::runtime_error(std::string(__func__) + ": New cached parent xpub does not match already cached parent xpub");
            }
            continue;
        }
        CacheParentExtPubKey(parent_xpub_pair.first, parent_xpub_pair.second);
        diff.CacheParentExtPubKey(parent_xpub_pair.first, parent_xpub_pair.second);
    }
    for (const auto& derived_xpub_map_pair : other.GetCachedDerivedExtPubKeys()) {
        for (const auto& derived_xpub_pair : derived_xpub_map_pair.second) {
            CExtPubKey xpub;
            if (GetCachedDerivedExtPubKey(derived_xpub_map_pair.first, derived_xpub_pair.first, xpub)) {
                if (xpub != derived_xpub_pair.second) {
                    throw std::runtime_error(std::string(__func__) + ": New cached derived xpub does not match already cached derived xpub");
                }
                continue;
            }