25The opposite result would undermine the principle that workers are to be covered by only one social security system, would make it difficult to know which system is applicable and would consequently impair legal certainty. In cases in which it was difficult to determine the system applicable, each of the competent institutions of the two Member States concerned would be inclined to take the view, to the detriment of the workers concerned, that their own social security system was applicable to them (FTS, paragraph 54).
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