As I understand it, both the prefixes "Hept-" and "Sept-" are used to indicate seven of something.
We have examples of English words that use both:
Heptathalon, Heptagon, Heptane
I don't believe this is the case for other numbers- "Quad" is four, "Pent" is five, "Hex" is six, etc...
I believe "Sept" comes from the word "septem", which is Latin for "seven". So where has "Hept" come from? And why do no other numbers have an equivalent?