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If the "slippers" are indeed thick rubberized shower shoes (like very substantial versions of beach flip flops), they can probably be sanitized and reused, but anything than looks like it cannot withstand boiling should be assumed to be new and disposable.
I once asked a hotel about this after seeing a confusing sign near the slippers that were provided by the hotel and was reassured that (1) the slippers were new and hadn't been used by a previous guest and (2) they were mine to keep if I so desired.