Try:
SELECT dbo.forum.Product
,dbo.forum.Supplier
,dbo.forum.SupplierCrossReference
,dbo.forum.DateLastQuoted
FROM dbo.forum
WHERE SupplierSequence = '500'
AND Product IN (SELECT Product
FROM dbo.Forum
WHERE SupplierSequence = '500'
GROUP BY Product
HAVING COUNT(1) > 1)
or
SELECT dbo.forum.Product
,dbo.forum.Supplier
,dbo.forum.SupplierCrossReference
,dbo.forum.DateLastQuoted
FROM dbo.forum
INNER JOIN (SELECT Product
,MAX(DateLastQuoted) DateLastQuoted
FROM dbo.Forum
WHERE SupplierSequence = '500'
GROUP BY Product
HAVING COUNT(1) > 1) p
ON p.Product = dbo.forum.Product
AND p.DateLastQuoted = dbo.forum.DateLastQuoted
WHERE dbo.forum.SupplierSequence = '500'