Right now it doesn’t seem to be ordered at all after the invoicedate.
It should also mark duplicate references.
The sort is only on the column for which you click the heading.
What do you mean by duplicate references? Do you mean the invoice number? If so, why should Manager mark duplicates? That field is overwritable so you can enter anything you want. Manager automatically provides a unique invoice number one higher than the highest existing one, even if you clone the invoice. The assumption is that if you manually entered something different, you had a reason. If you don’t want any duplicates, let Manager number invoices for you.
Sorting works. But if I have several invoices on the same day, the secondary ordering (nvoice number) is not there.
The reason I would like to see duplicates, is that I regularly make draft-invoices, so I don’t forget. That should have been my second request 
Try sorting on invoice number first, then sort by date. I think the ordering of the first step will be preserved.
This is not good accounting practice. Your accounting records should be a history of what has happened, reflecting events to date and your current financial position. No accounting system should be cluttered with what might happen, for the simple reason that it might not. Entering a draft invoice distorts your financial position in many ways.
Would an option that allows an invoice to be in its " draft" state be an option. In that case you can already make the invoice but it will not affect your final position. If you make it permanent it will affect your financial position; and by making it permanent I would suggest to lock the invoice, so no further changes can be made.
I can now make an invoice with a certain reference no, print it, file it, but change it later to my own benefit without leaving a paper trace in manager. If the invoice would be locked, I would have to make a new invoice or a credit invoice to alter something done in the past, thus leaving a paper trace…
Use a sales quote. That will not affect your financial position. If the sale is completed, convert it to a sales invoice. See Manager Cloud.
I understand that the sales quote does the trick in terms of making a document that does not affect the final position. That still does not alter my second point, where you can edit an invoice without leaving a paper trail.
I don’t understand your question, @patrickdewit. You edit the invoice by clicking the Edit button in the register. Once you click Update, that is your invoice. There is no record of changes made. And there is no “paper trail” as you call it.