Does any molecule other than DNA have a double-helical structure?
A few examples:

Starch
A polymer of glucose that can form a double helix and functions primarily as energy storage in plants.

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f-Actin
Filamentous actin forms a helical structure with two strands of polymerized g-actin. This is a structural component of the cytoskeleton.

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Coiled Coil
Protein motif with a helical structure formed by two (or more) α-helices. Coiled coils are found in a diverse range of proteins from structural proteins like keratin to transcription factors like c-Fos.

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Gramicidin
A peptide-based antibiotic that has been shown to form membrane spanning double helices.

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