Why do I get 2 hits with different data from PayPal's IPN?

Why do I get 2 hits with different data from PayPal's IPN?

After I make a simple web transaction on my site, my IPN handler recieves
not just the one expected 'completed' hit, but 2 hits. Each with different
data. The first looks like this:
stdClass Object
(
[mc_gross] => 50.00
[invoice] => PVIFmMOZTm2xkjbs
[protection_eligibility] => Ineligible
[payer_id] => NRQL7RKB54XWY
[tax] => 0.00
[payment_date] => 12:57:41 Sep 18, 2013 PDT
[payment_status] => Completed
[charset] => windows-1252
[first_name] => Ashley
[mc_fee] => 1.90
[notify_version] => 3.7
[custom] =>
[payer_status] => unverified
[business] => me@gmail.com
[quantity] => 0
[verify_sign] => AFcWxV21C7fd0v3bYYYRCpSSRl31AJGmBkDrCSx-mp-dKf5.Wc6co3ra
[payer_email] => payer@gmail.co.uk
[memo] => SOMETHING
[txn_id] => 08B29244UF9480324
[payment_type] => instant
[last_name] => Ward
[receiver_email] => me@gmail.com
[payment_fee] =>
[receiver_id] => XSSCKHZJZRBDQ
[txn_type] => web_accept
[item_name] =>
[mc_currency] => GBP
[item_number] =>
[residence_country] => GB
[test_ipn] => 1
[receipt_id] => 2722-5250-1650-6726
[transaction_subject] =>
[payment_gross] =>
[ipn_track_id] => a503d9ea64b1e
)
the second looks like this:
stdClass Object
(
[transaction] => Array
(
[0] => stdClass Object
(
[invoiceId] => PVIFmMOZTm2xkjbs
[id_for_sender_txn] => 3M760109N8943254L
[receiver] => me@gmail.com
[is_primary_receiver] => false
[id] => 08B29244UF9480324
[status] => Completed
[paymentType] => GOODS
[status_for_sender_txn] => Completed
[pending_reason] => NONE
[amount] => GBP 50.00
)
)
[payment_request_date] => Wed Sep 18 12:55:36 PDT 2013
[return_url] => http://mysite.com/checkout/paypal-overlay-state/complete
[fees_payer] => EACHRECEIVER
[ipn_notification_url] => http://mysite.com/ipn
[sender_email] => buyer@gmail.co.uk
[verify_sign] => AFcWxV21C7fd0v3bYYYRCpSSRl31ASUp2KT0F-R3d2QqGIzfWpkS.gWz
[test_ipn] => 1
[cancel_url] => http://mysite.com/checkout/paypal-overlay-state/cancel
[pay_key] => AP-4V279017Y7501703X
[action_type] => PAY
[memo] => SOMETHING
[transaction_type] => Adaptive Payment PAY
[tracking_id] => PVIFmMOZTm2xkjbs
[status] => COMPLETED
[log_default_shipping_address_in_transaction] => false
[charset] => windows-1252
[notify_version] => UNVERSIONED
[reverse_all_parallel_payments_on_error] => false
)
How do I make sense of this data? Why are there 2 hits? and which is the
one I need to look out for?
Thanks!