How to get the Total Net Weight for the Commercial Invoice without repeating the same weight?

I have created a Custom Commercial Invoice and there is a Net Weight(Kg) filed in it as follows. 

Corresponding to the above invoice there a two orders in the the detail section of the Sales Orders’ Invoices tab as follows.

There is only one order weight for both these items in the FREIGHT section as follows.  

If i use the =Sum[SOFreightDetail.Weight] for get the summation as the Net Weight(Kg) the weight is getting double. Please check the below screenshot.

Can I know If there is another different Order in the DETAILS section of the Sales Order Invoice Tab and if there is different order weight for that in the FREIGHT detail section, how to get the total net weight for the above invoice without repeating the same weight?