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In this paper we investigate the advanced unambiguous state discrimination (USD) attack that is crucial for quantum key distribution based on phase-coded coherent states. Performing the attack Eve imposes all discriminated states while she can maintain both detection and error rates at Bob's side. In this work we give the detailed description of the attack and propose the general approach for countermeasures. As the particular case we introduce the so-called'mousetrap'(uninformative) state which allows to decrease the USD probability and preserve high sifted key rate. We also propose some candidates for'mousetrap'states and provide the estimation on the discrimination probability for each of them.