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I'll give you a hint, and you can work out the details yourself. Take any $m_1,m_2,m_3$ of length $n$ (where $n$ is the block length), with $m_1\neq m_2$. Query the oracle with $m_1$, then query the oracle with $m_2$, and finally query the oracle with $m_1\|n\|m_3$. Work through this, and you can find a message and its forgery.

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