Hi Graeme,

Customer duplicates are created when the checkout without logging in is available on your website. One of the ways to avoid that is disabling of e-commerce purchasing without signing in.

In Odoo 18.0 you can configure it this way:

After that setting, your customers' website orders will be linked to the single partner. 

Best regards, Yurii Razumovskyi.

Company Garazd Creation — Odoo solutions for e-Commerce, SEO, Data Feeds, Website Tracking, Marketing, Analytics, and Marketplace integrations, Product Label designing and printing.

https://garazd.biz  |  Our solutions on Odoo Apps

Preventing Duplicate Contacts from Repeat E-commerce Customers in Odoo

1. Enforce Unique Identifiers
   • Ensure your website’s sign-up and checkout processes ask for both email and phone number, marking them as required.
   • Promote customer login rather than guest checkout so that Odoo can link orders to existing contacts automatically.
2. Configure Contact Matching Rules
   • In Settings → Contacts, enable and configure duplicate checking rules (e.g., “Email is the same” or “Name + Email”), which Odoo will enforce during contact creation.
3. Activate Auto-Merge or Warning Alerts
   • Choose whether Odoo should show a warning prompt when a duplicate contact is detected, or auto-merge based on specific fields (like exact email match). Enable this in the duplicate rules settings.
4. Encourage Existing User Login
   • On your website, clearly prompt customers to log into their account before placing an order.
   • Optionally, disable guest checkout to ensure all orders are tied to known contacts.
5. Clean Up Existing Duplicates
   • Use Odoo’s Merge Contacts tool to review and merge duplicate records in bulk, helping prevent further issues.
   • Consider scheduling periodic cleanups, especially after high-volume sales campaigns.
6. Use Third-party Tools for Extra Control
   • If you need more advanced deduplication (like fuzzy matching), look into community modules from the OCA (Odoo Community Association) or available in the Odoo App Store.