Open POS session and it will set the number for the next order (eg 1) 

Then duplicate the browser (right click the browser tab and duplicate) next tab also start with same order number (her its 1)

Now you have two tab with same order receipt reference number and continue sale in both the tabs, Finally only one tab order will be saved in backend other tab order will be missed.

Moreover you will have two pos receipt with same order reference. 

If any one have solution for this please help

https://github.com/odoo/odoo/issues/7388