41Therefore, for as long as an E 101 certificate is not withdrawn or declared invalid, the competent institution of the Member State in which an employee actually works must take account of the fact that that person is already subject to the social security legislation of the Member State in which the undertaking employing him is established, and that institution cannot therefore subject the worker in question to its own social security system (judgment of 27 April 2017, A-Rosa Flussschiff, C‑620/15, EU:C:2017:309, paragraph 43 and the case-law cited).
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