Somehow you've managed to create an entity that has two attributes with the same name.
There are a few ways to do this, but taking your confusion into account this is probably due to inheritance. If you create Customer, add an email attribute and then select a generalization entity that ALSO has an email attribute, the entity will consider this a duplicate. Simply because your vehicle happens to be a car doesn't mean it's any less a vehicle.
Check your generalization chain for an email attribute, remove one of the two and you should be fine.
Many thanks for your help & for pointing me in the right direction. Found the rogue email & corrected. On with the lectures!