Reference for Yevamot 73:13

וקמיפלגי בדרבי אלעזר דתנן רבי אלעזר אומר ודאן בודאן מותר ודאן בספקן וספקן בודאן וספקן בספקן אסור

while the second<span class="x" onmousemove="('comment',' Any child after the first, born from the levirate marriage. ');">²⁷</span> is one of doubtful bastardy<span class="x" onmousemove="('comment',' Cur. edd., 'There is no bastard on account of doubt'. ');">²⁹</span> and is consequently forbidden to marry a bastard.<span class="x" onmousemove="('comment',' Since it is equally possible that he himself is not a bastard. ');">³²</span>

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