Controller Update duplicate data

The problem lies in foreach loop. Try this:

foreach (var item in salesDetail)
{
    if (item.SalesDetailId > 0)
    {
        db.Entry(item).State = EntityState.Modified;
        retMessage.Message = "Update successfully!";
    }
    else
    {
        var newSale = new tblSale(); // Create a new instance for each sales detail
        newSale.tblSalesDetails.Add(new tblSalesDetail
        {
            ProductId = item.ProductId,
            UnitPrice = item.UnitPrice,
            Quantity = item.Quantity,
            LineTotal = item.LineTotal
        });
        var prd = db.tblProductStocks.Where(x => x.ProductId == item.ProductId && x.Quantity > 0).FirstOrDefault();
        prd.Quantity = prd.Quantity - item.Quantity;
        db.Entry(prd).State = EntityState.Modified;
        db.tblSales.Add(newSale); // Add the newSale instance to db.tblSales
        retMessage.Message = "Save successfully!";
    }
}

issue with your code is that you are adding the sale object to the db.tblSales table inside the foreach loop. This is causing the data to be duplicated every time the loop iterates. To fix this issue, you should move the db.tblSales.Add(sale) line outside the loop.

public JsonResult UpdateInvoiceSale(tblSale sale, List salesDetail, List deleted)
{
    MasterMVCPOS.Helper.AppHelper.ReturnMessage retMessage = new AppHelper.ReturnMessage();
    ASPNETMASTERPOSTEntities db = new ASPNETMASTERPOSTEntities();
    retMessage.IsSuccess = true;

    if (deleted != null)
    {
        foreach (var item in deleted)
        {
            var data = db.tblSalesDetails.FirstOrDefault(x => x.SalesDetailId == item);
            if (data != null)
            {
                db.tblSalesDetails.Remove(data);
            }
        }
    }

    foreach (var item in salesDetail)
    {
        if (item.SalesDetailId > 0)
        {
            db.Entry(item).State = EntityState.Modified;
            retMessage.Message = "Update successfully!";
        }
        else
        {
            sale.tblSalesDetails.Add(new tblSalesDetail
            {
                ProductId = item.ProductId,
                UnitPrice = item.UnitPrice,
                Quantity = item.Quantity,
                LineTotal = item.LineTotal
            });

            var prd = db.tblProductStocks.FirstOrDefault(x => x.ProductId == item.ProductId && x.Quantity > 0);
            if (prd != null)
            {
                prd.Quantity = prd.Quantity - item.Quantity;
                db.Entry(prd).State = EntityState.Modified;
            }

            retMessage.Message = "Save successfully!";
        }
    }

    try
    {
        db.tblSales.Add(sale); // Move this line outside the loop
        db.SaveChanges();
    }
    catch (Exception)
    {
        retMessage.IsSuccess = false;
    }

    return Json(retMessage, JsonRequestBehavior.AllowGet);
}

n this modified code, the db.tblSales.Add(sale) line is moved outside the loop to ensure that the sale object is added to the database only once. This should prevent the duplication of data when updating saved records.

I made a few other improvements to your code, such as checking for null values before removing or modifying data and using the FirstOrDefault() method instead of FirstOrDefault() in the appropriate places.