Below are the error message and the dumps of the query objects from the two attempts to set access in the node_access table.

ERROR MESSAGE:
PDOException: SQLSTATE[23000]: Integrity constraint violation: 1062 Duplicate entry '17-4-nodeaccess_userreference' for key 'PRIMARY': INSERT INTO {node_access} (nid, realm, gid, grant_view, grant_update, grant_delete) VALUES (:db_insert_placeholder_0, :db_insert_placeholder_1, :db_insert_placeholder_2, :db_insert_placeholder_3, :db_insert_placeholder_4, :db_insert_placeholder_5), (:db_insert_placeholder_6, :db_insert_placeholder_7, :db_insert_placeholder_8, :db_insert_placeholder_9, :db_insert_placeholder_10, :db_insert_placeholder_11), (:db_insert_placeholder_12, :db_insert_placeholder_13, :db_insert_placeholder_14, :db_insert_placeholder_15, :db_insert_placeholder_16, :db_insert_placeholder_17); Array ( [:db_insert_placeholder_0] => 17 [:db_insert_placeholder_1] => nodeaccess_userreference [:db_insert_placeholder_2] => 4 [:db_insert_placeholder_3] => 1 [:db_insert_placeholder_4] => 0 [:db_insert_placeholder_5] => 0 [:db_insert_placeholder_6] => 17 [:db_insert_placeholder_7] => nodeaccess_userreference [:db_insert_placeholder_8] => 1 [:db_insert_placeholder_9] => 1 [:db_insert_placeholder_10] => 0 [:db_insert_placeholder_11] => 0 [:db_insert_placeholder_12] => 17 [:db_insert_placeholder_13] => nodeaccess_userreference_author [:db_insert_placeholder_14] => 2 [:db_insert_placeholder_15] => 1 [:db_insert_placeholder_16] => 1 [:db_insert_placeholder_17] => 1 ) in node_access_write_grants() (line 3440 of /Users/ghostlyTrickster/Sites/drupal3/modules/node/node.module).

Dump of query object:
InsertQuery_mysql Object
(
    [table:protected] => node_access
    [insertFields:protected] => Array
        (
            [0] => nid
            [1] => realm
            [2] => gid
            [3] => grant_view
            [4] => grant_update
            [5] => grant_delete
        )

    [defaultFields:protected] => Array
        (
        )

    [insertValues:protected] => Array
        (
            [0] => Array
                (
                    [0] => 23
                    [1] => nodeaccess_userreference
                    [2] => 4
                    [3] => 1
                    [4] => 0
                    [5] => 0
                )

            [1] => Array
                (
                    [0] => 23
                    [1] => nodeaccess_userreference
                    [2] => 1
                    [3] => 1
                    [4] => 0
                    [5] => 0
                )

            [2] => Array
                (
                    [0] => 23
                    [1] => nodeaccess_userreference_author
                    [2] => 2
                    [3] => 1
                    [4] => 1
                    [5] => 1
                )

        )

    [fromQuery:protected] =>
    [connection:protected] => DatabaseConnection_mysql Object
        (
            [shutdownRegistered:protected] =>
            [target:protected] => default
            [key:protected] => default
            [logger:protected] =>
            [transactionLayers:protected] => Array
                (
                    [drupal_transaction] => drupal_transaction
                    [savepoint_1] => savepoint_1
                )

            [driverClasses:protected] => Array
                (
                    [SelectQuery] => SelectQuery
                    [DatabaseTransaction] => DatabaseTransaction
                    [InsertQuery] => InsertQuery_mysql
                    [UpdateQuery] => UpdateQuery
                    [MergeQuery] => MergeQuery
                    [DeleteQuery] => DeleteQuery
                )

            [statementClass:protected] => DatabaseStatementBase
            [transactionSupport:protected] => 1
            [transactionalDDLSupport:protected] =>
            [temporaryNameIndex:protected] => 0
            [connectionOptions:protected] => Array
                (
                    [database] => lilacpro_drupwlk
                    [username] => drupal3
                    [password] => wl0wl0wl000
                    [host] => localhost
                    [port] =>
                    [driver] => mysql
                    [prefix] => Array
                        (
                            [default] =>
                        )

                )

            [schema:protected] =>
            [prefixes:protected] => Array
                (
                    [default] =>
                )

            [prefixSearch:protected] => Array
                (
                    [0] => {
                    [1] => }
                )

            [prefixReplace:protected] => Array
                (
                    [0] =>
                    [1] =>
                )

        )

    [connectionTarget:protected] => default
    [connectionKey:protected] => default
    [queryOptions:protected] => Array
        (
            [target] => default
            [return] => 3
        )

    [uniqueIdentifier:protected] => 50edab60008ef7.00761431
    [nextPlaceholder:protected] => 0
    [comments:protected] => Array
        (
        )

)


NOTE: The second query is exactly the same as the first, except that the uniqueIdentifier is different, and connection->transactionlayers['savepoint1'] is missing.