This question from introduction to modern cryptography book

Show how to find a collision in the Merkle tree construction if t is not fixed. Specifically, show how to find two sets of inputs x₁, ..., x_t and x₁, ..., x_2t, such that MT_t(x₁, ..., x_t) = MT_2t(x₁, ..., x_2t).

Can you design an approach to fix the problem and have variable-length collision resistant Merkle tree?