Reference for Shevuot 7:1

אינו חייב אלא אחת זו היא שבועת ביטוי שחייבין על זדונה מכות ועל שגגתה קרבן עולה ויורד

he is guilty of transgressing only one oath:<span class="x" onmousemove="('comment',' The first: for, having uttered the first oath, the loaf is already prohibited to him; and when he utters the second oath, he is, as it were, swearing to fulfil a mizvah [i.e., to fulfil the first oath]; and he who swears to fulfil a mizvah, and does not fulfil it, is not liable to punishment; v. infra 27a');">¹</span> this is the 'useless oath'<span class="x" onmousemove="('comment',' See Lev. V, 4.');">²</span>

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