Left Join Insert Query Error

I have been working on this for a little bit and have gotten to this point. Below is the query in blue, the error in red. Now, from what I gather the error is telling me I can't insert a duplicate product code into tblProduct. Isn't that what the left join is exactly not doing, as in, the left join is inserting all records from Complete_records into tblProduct where the code is null(does not exist) in tblProduct? Or, is this an issue where the identity number, productID in tblProduct, isn't starting at the next number in turn?

Thanks.

Set Identity_Insert tblProduct on
DECLARE @MaxId int

SELECT @MaxID=MAX(productID)
FROM tblProduct

SELECT IDENTITY(int,1,1) AS ID,Complete_products.APNum, Complete_products.Title, Complete_products.CategoryID, Complete_products.Mountable, Complete_products.price,
Complete_products.Height, Complete_products.Width, Complete_products.IRank, Complete_products.frameable, Complete_products.Typ INTO #Temp
FROM Complete_products LEFT OUTER JOIN
tblProduct AS tblProduct_1 ON Complete_products.APNum = tblProduct_1.productCode
where tblProduct_1.productCode IS NULL

INSERT INTO tblProduct
(productID,productCode, productName, productNavID, CanBeMounted, productRetailPrice, productHeight, productWidth, Rank, CanBeFramed, ProductType)
SELECT @MaxID + ID, APNum, Title, CategoryID, Mountable, price,
Height, Width, IRank, frameable, Typ
FROM #Temp

(236752 row(s) affected)
Msg 2601, Level 14, State 1, Line 13
Cannot insert duplicate key row in object 'dbo.tblProduct' with unique index 'IX_tblProductt_productCode'.
The statement has been terminated.