DNA is known to have a double-helical structure. Do any other molecules have this structure?
A few examples:
A polymer of glucose that can form a double helix and functions primarily as energy storage in plants.
Filamentous actin forms a helical structure with two strands of polymerized g-actin. This is a structural component of the cytoskeleton.
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.
A peptide-based antibiotic that has been shown to form membrane spanning double helices.
The structural protein collagen consists of a triple helix of polypeptides. Whether this answers the question is arguable—you could say that the triple helix contains double helices. In any case, depending on what prompted your question, I thought it might be an interesting structure to consider.