This appears to be a bug in Zohobooks. It wrongly allows for the existence of multiple contacts with the same email address, and it wrongly uses case-sensitive comparisons for email-addresses when looking up contacts, email addresses should always be treated as case-insensitive!

For example:

Paul Smith            psmith@sample.com

Paul Smith 2         psmith@sample.com

Paul Smith 3         PSmith@sample.com

All of above could be in Zohobooks, it's anyone's guess what the API at https://www.zoho.com/books/api/v3/contacts/#list-contacts (with query param email=psmith@sample.com) would return!

Can someone please clarify, and fix this bug? Email addresses need to be treated as case-insensitive entries, and as such must be unique!