Hello,
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Do not worry, as you got the marksheet from Institute itself, your will accepted for admissions with the duplicate sheet. Duplicate in means the copy of mark's sheet. Before the admissions even clear the doubt from Institute whether you can get original memo or not. If they can provide you original via any particular process go through it, if  they can't, go for admission via duplicate sheet. Even get prepared for a strong reasons, as few of the colleges may ask. So, hope everything goes fine. Best of luck