Chicago Manual of Style DOI
The Chicago Manual of Style (Author-Date) template in Zotero automatically prefixes all DOI's with http://dx.doi.org when generating bibliographies. According to Rule 15.9 of the Chicago Manual of Style, the proper format for listing a DOI is to add at the end of the bibliography item "doi:" in all lowercase and no space between the colon and the DOI number. The manual makes no reference to including the URL that Zotero automatically inserts.
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Liu, Jui-Ch’i. 2015. “Beholding the Feminine Sublime: Lee Miller’s War Photography.” Signs 40, no. 2 (Winter): 308–19. https://doi.org/10.1086/678242.
(This is also in line with universally agreed upon DOI display guidelines, btw.)