This is a metabug since this issue (which is tracked by 483) is now fixed when using Code First. The fix for this when using Database First or Model First will be post-EF6.
A canonical example of this bug is having a Product entity that is included in your model and then a separate Product DTO that is defined in a different namespace. In EF5 and earlier we would throw because EF couldn't work out which Product class to use...
but in the case of Code First we know which one to use because you define you model in terms of your CLR classes.
See item 483 for more details: