Index: wp-admin/includes/file.php
===================================================================
--- wp-admin/includes/file.php	(revision 43298)
+++ wp-admin/includes/file.php	(working copy)
@@ -2347,8 +2347,23 @@
 			$groups[ $group_id ]['items'][ $item_id ] = array();
 		}
 
-		$old_item_data = $groups[ $group_id ]['items'][ $item_id ];
-		$merged_item_data = array_merge( $export_datum['data'], $old_item_data );
+		$merged_item_data = $groups[ $group_id ]['items'][ $item_id ];
+		//append the new data, but don't overwrite the data that has already been collected
+		foreach ( $export_datum['data'] as $new_key => $new_value ) {
+			if ( is_int( $new_key ) ) {
+				//append numeric indices, similar to the array_merge functionality
+				$merged_item_data[] = $new_value;
+				continue;
+			}
+
+			if ( array_key_exists( $new_key, $merged_item_data ) ) {
+				//avoid overwriting the existing higher priority data with lower priority
+				continue;
+			}
+
+			$merged_item_data[ $new_key ] = $new_value;
+		}
+
 		$groups[ $group_id ]['items'][ $item_id ] = $merged_item_data;
 	}