When I try to edit a contact to add an email address it will not allow it. The error message says it’s trying to prevent me adding a duplicate contact.

Browser's cache and cookies can cause QuickBooks Solopreneur's performance to change when modifying a contact's information, Nick. Therefore, we can perform basic troubleshooting to rectify this. I'll walk you through the process to sort this out promptly.

Firstly, open your company file in an incognito or private browser. You can quickly launch this by using the following shortcut keys:

• Safari 11 or newer: ⌘+Shift+N
• Chrome: Ctrl+Shift+N
• Firefox: Ctrl+Shift+P
• Safari: Command+Shift+N

Once you're in, add the new email address to the contact. If it goes through, proceed to clear your browser's cache. Refer to this article for the steps: Clear cache and cookies to fix issues when using QuickBooks Online.

You can also consider using another supported browser when modifying the email address of a contact in QuickBooks. For the list of browsers, please see this article: System requirements for QuickBooks Online products.

On the other hand, here's an article to learn more about the capabilities of the program: Introduction to QuickBooks Solopreneur.

If you also need assistance dealing with customer transactions or performing specific tasks, please let me know in the comment section. I'll make it my top priority to provide you with the details needed to address your inquiries. Keep safe.