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I don't believe I've ever had a double cover and I've been making ebooks since 2011. But if you're using Sigil to edit your epub for uploading to Kindlegen/KindlePreviewer, it is important to get the Guide items correct (Add Semantics - cover image, title page, TOC, text). And I use a frontispiece often or an image as a title page... never had a problem. Just be sure you Add Semantics - Title Page for that with Sigil.
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