premise: The S-99 (Russian: С-99 ) experimental submarine was the only ship of the Soviet Project 617 submarine class (NATO reporting name: Whale class) that the Soviet Union built during the early Cold War and the only Soviet submarine which had a Walter engine fuelled by high test peroxide (HTP).
hypothesis: The Soviet Union used a Walter engine fuelled by high test peroxide (HTP) only once - for the experimental submarine S-99 (Russian: С-99).
reason: Based on the information provided, the S-99 (Russian: С-99) is the only Soviet submarine which had a Walter engine fuelled by high test peroxide (HTP). However, we don't know if the Soviets used the Walter engine for other projects (not submarines), therefore, the statement that they used it only once is neither correct, nor incorrect. 