Prove that the following MAC is insecure I am trying to prove that the following MAC is insecure, but I don't know how to exploit the properties of the pseudorandom function $F$ involved:

Gen generates a uniform $k \in \{0, 1\}^n$.

To authenticate a message $m_1 || m_2$ with $|m_1| = |m_2| = n$, compute the tag $F_k(m_1)||F_k(F_k(m_2))$.

Any help?
I won't give the answer to homework questions, but I will give a hint.

Suppose you learn the tags for $m_1 || m_2$ and $m_3 || m_4$; what other messages could you deduce the tags for?