No receipt sent for PayPal Pro Recurring Donations

Using PayPal Pro's recurring donation service as the processor, when a recurring donation "ticks", including for the first time, the result is recorded correctly, and CiviCRM claims that it has sent a receipt. However, no receipt is actually sent.

Looking at the logs, CiviCRM posts the array for the donation, records that "Contribution record was updated successfully", then posts the same array a second time, and spits out the following error:

Jun 27 07:47:02 [info] $Fatal Error Details = Array
(
[callback] => Array
(
[0] => CRM_Core_Error
[1] => handle
)

[code] => -5
[message] => DB Error: already exists
[mode] => 16
[debug_info] => INSERT INTO civicrm_contribution (contact_id , financial_type_id , contribution_page_id , payment_instrument_id , receive_date , total_amount , fee_amount , net_amount , trxn_id , invoice_id , currency , cancel_date , receipt_date , thankyou_date , source , contribution_recur_id , contribution_status_id ) VALUES ( 52 , 1 , 1 , 1 , 20130627074702 , 10.00 , 0.59 , 9.41 , '97286874E5998541T' , 'fc075fa545460447df8e06a63df178a7' , 'USD' , NULL , 20130627074702 , NULL , 'Donation Page' , 7 , 1 ) [nativecode=1062 ** Duplicate entry '97286874E5998541T' for key 'UI_contrib_trxn_id']
[type] => DB_Error
[user_info] => INSERT INTO civicrm_contribution (contact_id , financial_type_id , contribution_page_id , payment_instrument_id , receive_date , total_amount , fee_amount , net_amount , trxn_id , invoice_id , currency , cancel_date , receipt_date , thankyou_date , source , contribution_recur_id , contribution_status_id ) VALUES ( 52 , 1 , 1 , 1 , 20130627074702 , 10.00 , 0.59 , 9.41 , '97286874E5998541T' , 'fc075fa545460447df8e06a63df178a7' , 'USD' , NULL , 20130627074702 , NULL , 'Donation Page' , 7 , 1 ) [nativecode=1062 ** Duplicate entry '97286874E5998541T' for key 'UI_contrib_trxn_id']
[to_string] => [db_error: message="DB Error: already exists" code=-5 mode=callback callback=CRM_Core_Error::handle prefix="" info="INSERT INTO civicrm_contribution (contact_id , financial_type_id , contribution_page_id , payment_instrument_id , receive_date , total_amount , fee_amount , net_amount , trxn_id , invoice_id , currency , cancel_date , receipt_date , thankyou_date , source , contribution_recur_id , contribution_status_id ) VALUES ( 52 , 1 , 1 , 1 , 20130627074702 , 10.00 , 0.59 , 9.41 , '97286874E5998541T' , 'fc075fa545460447df8e06a63df178a7' , 'USD' , NULL , 20130627074702 , NULL , 'Donation Page' , 7 , 1 ) [nativecode=1062 ** Duplicate entry '97286874E5998541T' for key 'UI_contrib_trxn_id']"]
)

Jun 27 07:47:02 [info] $backTrace = #0 /home/website/public_html/wp-content/plugins/civicrm/civicrm/CRM/Core/Error.php(196): CRM_Core_Error::backtrace("backTrace", TRUE)
#1 [internal function](): CRM_Core_Error::handle(Object(DB_Error))
#2 /home/website/public_html/wp-content/plugins/civicrm/civicrm/packages/PEAR.php(931): call_user_func((Array:2), Object(DB_Error))
#3 /home/website/public_html/wp-content/plugins/civicrm/civicrm/packages/DB.php(969): PEAR_Error->PEAR_Error("DB Error: already exists", -5, 16, (Array:2), "INSERT INTO civicrm_contribution (contact_id , financial_type_id , contributi...")
#4 /home/website/public_html/wp-content/plugins/civicrm/civicrm/packages/PEAR.php(564): DB_Error->DB_Error(-5, 16, (Array:2), "INSERT INTO civicrm_contribution (contact_id , financial_type_id , contributi...")
#5 /home/website/public_html/wp-content/plugins/civicrm/civicrm/packages/DB/common.php(1905): PEAR->raiseError(NULL, -5, NULL, NULL, "INSERT INTO civicrm_contribution (contact_id , financial_type_id , contributi...", "DB_Error", TRUE)
#6 /home/website/public_html/wp-content/plugins/civicrm/civicrm/packages/DB/mysql.php(898): DB_common->raiseError(-5, NULL, NULL, NULL, "1062 ** Duplicate entry '97286874E5998541T' for key 'UI_contrib_trxn_id'")
#7 /home/website/public_html/wp-content/plugins/civicrm/civicrm/packages/DB/mysql.php(327): DB_mysql->mysqlRaiseError()
#8 /home/website/public_html/wp-content/plugins/civicrm/civicrm/packages/DB/common.php(1216): DB_mysql->simpleQuery("INSERT INTO civicrm_contribution (contact_id , financial_type_id , contributi...")
#9 /home/website/public_html/wp-content/plugins/civicrm/civicrm/packages/DB/DataObject.php(2421): DB_common->query("INSERT INTO civicrm_contribution (contact_id , financial_type_id , contributi...")
#10 /home/website/public_html/wp-content/plugins/civicrm/civicrm/packages/DB/DataObject.php(1055): DB_DataObject->_query("INSERT INTO civicrm_contribution (contact_id , financial_type_id , contributi...")
#11 /home/website/public_html/wp-content/plugins/civicrm/civicrm/CRM/Core/DAO.php(287): DB_DataObject->insert()
#12 /home/website/public_html/wp-content/plugins/civicrm/civicrm/CRM/Core/Payment/BaseIPN.php(464): CRM_Core_DAO->save()
#13 /home/website/public_html/wp-content/plugins/civicrm/civicrm/CRM/Core/Payment/PayPalProIPN.php(273): CRM_Core_Payment_BaseIPN->completeTransaction((Array:17), (Array:9), (Array:8), Object(CRM_Core_Transaction), TRUE)
#14 /home/website/public_html/wp-content/plugins/civicrm/civicrm/CRM/Core/Payment/PayPalProIPN.php(212): CRM_Core_Payment_PayPalProIPN->single((Array:17), (Array:9), (Array:8), TRUE, FALSE)
#15 /home/website/public_html/wp-content/plugins/civicrm/civicrm/CRM/Core/Payment/PayPalProIPN.php(335): CRM_Core_Payment_PayPalProIPN->recur((Array:17), (Array:9), (Array:8), FALSE)
#16 /home/website/public_html/wp-content/plugins/civicrm/civicrm/extern/ipn.php(60): CRM_Core_Payment_PayPalProIPN->main("contribute")
#17

{main}