Code

void OnValidateAnchorPoint(object sender, LinkValidationEventArgs e)
{
	e.Cancel = CountLinks(e.Node, e.AnchorIndex) > 0;
}

int CountLinks(DiagramNode node, int anchorPoint)
{
	return
		node.IncomingLinks.Count(link => link.DestinationAnchor == anchorPoint) +
		node.OutgoingLinks.Count(link => link.OriginAnchor == anchorPoint);
} 

Code

void OnValidateAnchorPoint(object sender, LinkValidationEventArgs e)
{
	e.Cancel = CountLinks(e) > 0;
}

int CountLinks(LinkValidationEventArgs e)
{
	var link = e.Link;
	if (e.ChangingOrigin)
	{
		return e.Node.GetAllLinks().Count(l => l != link &&
			SameOrgAndDest(l, e.Node, e.AnchorIndex, link.Destination, link.DestinationAnchor));
	}
	else
	{
		return e.Node.GetAllLinks().Count(l => l != link &&
			SameOrgAndDest(l, link.Origin, link.OriginAnchor, e.Node, e.AnchorIndex));
	}
}

bool SameOrgAndDest(DiagramLink link,
	DiagramNode originNode, int originAnchor,
	DiagramNode destNode, int destAnchor)
{
	return
		originNode == link.Origin &&
		originAnchor == link.OriginAnchor &&
		destNode == link.Destination &&
		destAnchor == link.DestinationAnchor;
}