I wasn't able to do a left join due to the Memo error or a type conversion but this SQL will insert from a .CSV file into a SharePoint list without duplicates.
INSERT INTO Stock ( Description, Supplier, Qty, Unit_Cost )
SELECT Stockcsv.[(Cost) Description], Stockcsv.[(Cost) Supplier], Stockcsv.[(Cost) Quantity returned to Inventory], Stockcsv.[(Cost) Cost]
FROM [Text;FMT=Delimited;HDR=YES;CharacterSet=437); DATABASE=D:\Warehouse].Stock.csv AS Stockcsv
WHERE NOT EXISTS (SELECT * FROM Stock where Stock.Supplier = Stockcsv.[(Cost) Supplier] AND Stock.Description = Stockcsv.[(Cost) Description] AND Stock.Unit_Cost = Stockcsv.[(Cost) Cost] AND Stock.Qty = Stockcsv.[(Cost) Quantity returned to Inventory]);
The syntax is:
INSERT INTO destable ( field1,field2)
SELECT filestempname.field1, filestempname.field2
FROM [Text;FMT=Delimited;HDR=YES;CharacterSet=437); DATABASE=C:\your\file\path].filename.csv AS filestempname
WHERE NOT EXISTS (SELECT * FROM destable WHERE destable.field1 = filestempname.field1 AND destable.field2 = filestempname.field2);