Reference for Keritot 29:35

אינו חייב אלא אחת

The Sages, however, agree with R'Eliezer that if a man had intercourse at the same time<span class="x" onmousemove="('comment',' I.e. under one spell of unawareness. Rashi omits: 'at the same time'.');">¹⁹</span>

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