Hello, Try this module: Odoo all import for Sale, Purchase, Invoice, Inventory, Product and Customer.
This will definitely help you out.
For any query drop me a mail at: shashikantmane@browseinfo.in

I'm able to import Sales Order/Quotation and able to import it multiple times without creating duplicate Sales Order however the Line Items is different, It adds new lineitems everytime I import.

Is there some document that maybe helpful? Or better if somebody can provide a sample Sales Order Import File. I tried to experiment on External ID's for lineitems but so far it's NOT working.

Hello,

There is one app available on odoo store which will helps you to import sales order line easily. Here is the highlights of this app.

• Manage Large Sale Order through Excel

• Import/Export/Mass Update Order lines

• Import/Export Order lines from Excel popup

• Import file with simple Code & Qty

• Setting default Qty while importing products from Excel file

• Export the file and do mass update on order lines