Investigate if it's possible to end up with duplicate pauli strings when matrix-multiplying PauliOp with PauliString

[stand-by]

Investigate if it's possible to have duplicated pauli strings in resulting operator when matrix-multiplying PauliOp with PauliString.
If so, then implement de-duplication for functions like friend PauliOp<T, H> operator*(PauliOp<T, H> const &pauli_op_left, PauliString const &pauli_str_right) and friend PauliOp<T, H> operator*(PauliString const &pauli_str_left, PauliOp<T, H> const &pauli_op_right)