Why isn't E' secure if it's built over a secure cipher?
Summarizing the answer from the comments:

what happens if E is a stream cipher? ~ Richie Frame
What if you use exclusive-or to combine the key stream instead of addition? ~ Ella Rose
So $c_i=m_i\oplus k_i\oplus k_i=m_i$, and it would be trivially broken. Thanks! I wouldn't come up with the idea of using a stream cipher nor modifying the one-time pad like this. ~ Daniel