Suggested solution:

Get the received sequence number before the ack is made i.e.

function republish5105Err006() {
var subscription = new Subscription();
CreateSubscriptionHelper.Execute( { Subscription: subscription } );

// create monitored items
if( SubscriptionMonitoredItems.length === 0 ) {
    addSkipped( "Static Scalar - 2 Nodes needed" );
    return( false );
}

if( !CreateMonitoredItemsHelper.Execute( { ItemsToCreate: SubscriptionMonitoredItems, TimestampsToReturn: TimestampsToReturn.Both, SubscriptionId: subscription } ) ) return( false );

PublishHelper.WaitInterval( { Items: SubscriptionMonitoredItems, Subscription: subscription } );

// call Publish() to get the first sequence number
if( !PublishHelper.Execute() ) {
    DeleteSubscriptionsHelper.Execute( { SubscriptionIds: subscription } );
    return( false );
}

var FirstReceivedSequenceNumber = PublishHelper.ReceivedSequenceNumbers.pop();

// call Publish()() to acknowledge the first sequence
PublishHelper.Execute();

// call republish with the sequence number received above
RepublishHelper.Execute( { RetransmitSequenceNumber: FirstReceivedSequenceNumber, SubscriptionId: subscription, ServiceResult: new ExpectedAndAcceptedResults( [ StatusCode.BadMessageNotAvailable, StatusCode.BadMessageNotAvailable ] ) } );

// clean-up
DeleteMonitoredItemsHelper.Execute( { ItemsToDelete: SubscriptionMonitoredItems, SubscriptionId: subscription } );
DeleteSubscriptionsHelper.Execute ( { SubscriptionIds: subscription } );
return( true );

}