How do I duplicate an existing invoice?? This is to save inputting everything again and again for regular invoices.

Hi there, @usertim.

Allow me to share some details about creating a duplicate copy of invoices.

This option is currently unavailable. However, you can create a recurring transaction if you want to send your customer an invoice with the same details. This process is called Recurring Invoice. Note that this can only be setup to one customer only in every transaction.

Here's how to set up a Recurring invoice:

1. Select the + New button and select Enable recurring payments.
2. Enter the necessary information on the invoice form such as the Template name, Interval, and not limited to the Terms.
3. Once done, select Save and done or Save and send. This is how it looks like:

Also, if  you want to see how many times it was sent to the customer, or you want to make some changes about the details, you can go to the Settings and modify it. I'll show you the way.

1. Go to the Settings or Gear icon.
2. Select Recurring transactions under Lists column.
3. Look for the transaction and click edit to modify the information in the invoice. Or select the drop-down arrow if you want to skip next date, pause, or create a duplicate. Here's some screenshots for additional reference:

To know more details on how to manage  your recurring transactions, you can check out this article: How to create a recurring invoice and manage recurring transactions.

I've added this reference page in case you need to track you business financials: Financial reports in QuickBooks Online.

You're always welcome to tag my name in the comment section if you have other questions. Stay safe and have a great day!