Multiple Contacts with same phone number

[merlin266]

I am relatively new to the Android opsys. I am using the standard Contacts app on my Samsung Galaxy S4. I have noticed that if two contacts have the same phone number or same email address, only one of the Contacts is shown. I have tested this by changing the phone number in one contact by a single digit. When I do that, both contacts are visible. When I change back to the same phone number, one contact disappears. Ditto for the same email address. Is this normal?