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Preheat oven to 350 degrees. Butter and flour 12-cup Bundt pan.
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Combine flour, baking powder, soda, and salt; set aside.
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With mixer at high speed beat butter until fluffy, about 2 minutes. Beat in granulated sugar until fluffy. Reduce speed to medium; beat in eggs, one at a time, until combined, about 3 minutes. Reduce speed to low; beat in 1/2 cup lemon juice and zest. Beat in flour mixture alternately with buttermilk until combined; stir in poppy seeds. Increase speed to high; beat 1 minute. Pour into pan.
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Bake until toothpick inserted into center comes out clean, about 1 hour. Run knife between cake and pan to loosen; cool in pan on rack 25 minutes. Remove from pan; cool completely on rack.
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Combine confectioners' sugar with remaining lemon juice. Pour over cake. Just before serving, fill center with berries.