I wanted to export a customer list from Quickbooks, update then import aagain

You're correct that there are differences in the format between the exported and imported data, Steve.

When you export a customer list from QuickBooks, you will receive both the customer's full name and their display name. The customer display name is shown in the "Customer" column, while the full name is in the "Full Name" column:

However, when you import this data back into QuickBooks, the import function has specific limitations and there's also a specific format. Here's a screenshot for the correct format: 

During the import mapping process, you’ll notice that there is no dedicated field for the customer display name. Instead, you can see the  "Name" field:

This means that the import will not recognize the customer’s display name separately. Instead, you can only choose either the display name or the full name which will be populated into the "Full Name" field, and the original customer display names might be lost in the process.

While it may be time-consuming, manually updating each customer’s display name in QuickBooks would ensure that you accurately add the desired postcode or webshop customer IDs. Alternatively, you can add another column and name it postcode, then enter the associated postcode for each customer. During the mapping process, you can select this column and it will be imported accordingly. 

On top of that, you can consider using a third-party application or integration that allows you to manage customer data in bulk while preserving custom fields like the display name.
 

You can check out this article to learn how to send summaries of your customers invoices, payments, credits, and balances to them: Create and send customer statements in QuickBooks Online.

Let me know if you have further concerns. We'll be glad to help. 

Currently we are manually inputting all new customer data from our website.

What would happen if we started exporting the correct customer data from my website and tried importing it,  in the correct format csv, into Quickbooks if there was a duplicate customer, or even a customer with the same name but different address and contact details.

How does Quickbooks know what to override and what to create as a new customer?

1. Go to Sales and choose Customers.
2. Select the checkbox next to each customer you want to make inactive.
3. In the Batch actions dropdown, tap Make inactive.
4. Press Yes to confirm.