You can clone with
# this shows how to use the update functionality for a single feed with feedzirra
# I'm using Atom here, but it could be anything. You don't need to know ahead of time.
# It will parse out to the correct format when it updates.
feed_to_update = Feedzirra::Parser::Atom.new
feed_to_udpate.feed_url = some_stored_feed_url
feed_to_update.etag = some_stored_feed_etag
feed_to_update.last_modified = some_stored_feed_last_modified
last_entry = Feedzirra::Parser::AtomEntry.new
last_entry.url = the_url_of_the_last_entry_for_a_feed
feed_to_update.entries = [last_entry]
updated_feed = Feedzirra::Feed.update(feed_to_update)
updated_feed.updated? # => nil if there is nothing new
updated_feed.new_entries # =>  if nothing new otherwise a collection of feedzirra entries
updated_feed.etag # => same as before if nothing new. although could change with comments added to entries.
updated_feed.last_modified # => same as before if nothing new. although could change with comments added to entries.