Is it possible to empty the customer field when cloning an invoice to avoid accidentally invoicing the same customer again?
a clone option is intended to copy everything. else it would have been just called a template.
Is there then an easier way to do this:
I need to create invoices that usually have the same items (only changing every now and then). At the moment I clone a similar invoice and then change the due date and customer. It works ok, except every so often I tend to forget changing the customer, then have to backtrack at a later stage and figure out which customer it was actually intended for.
set the necessary details in Form Defaults under Settings.
https://forum.manager.io/t/set-form-defaults/14059
after setting all the defaults necessary, make sure you remove the customer name before you click Update.
That is the best news. Downloaded the latest version (v18) to access Form Defaults - works a treat. Just made a “NULL” customer as a workaround, just what I needed.
Thanks so much! 
There is no need for the NULL customer. Just delete the customer on the form default before updating it, as @sharpdrivetek suggested.
Ah! I missed that second part when I followed the link. That’s fantastic! Thanks so much for the help - really appreciate it!