Adding contacts iOS vs Android
It's kind of strange that you add contacts in iOS with their email address and on android with their phone number. So I got some doubled contacts on my phone.
Sorry for the confusion! You should be able to add contacts by either email address or phone number* on both platforms. Is it possible that you have different contact book info on your devices (e.g. maybe your iPhone has someone’s email address on it, but your Android device only has their phone number)?
*Adding contact by phone number may be disabled or modified in some countries where SMS invites are legally restricted.
Fabian Hertwig commented
When I select a friend in the contact book, a dark bar at the bottom opens up which asks "where should we send the invite?" And in the text field it says "enter an email address". I'm based in Germany.
The doubled contacts are the ones I added by mail and a friend added to a group by phone number.