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9 He knowing that the children should not be his, when he went in to his brother's wife, spilled his seed upon the ground, lest children should be born in his brother's name.

16 Thou shalt not uncover the nakedness of thy brother's wife: because it is the nakedness of thy brother.

Search the Scriptures: there is no contradiction as the latter refers to adultery which only occurs while the other spouse is alive:

For the woman which hath an husband is bound by the law to her husband so long as he liveth; but if the husband be dead, she is loosed from the law of her husband. So then if, while her husband liveth, she be married to another man, she shall be called an adulteress: but if her husband be dead, she is free from that law; so that she is no adulteress, though she be married to another man. (Romans 7:2-3)