I'm still [reading] trying to understand what bitcoins are.
Are they a just information on a hard drive?
And,
can they be backed up, or is it only possible
to have once instance of a bitcoin, like a paper dollar, or gold coin?
Well, it is a lots of things, not all of them were intentional:
-payment tool (push version, no refund type)
-digital currency
-an illiquid stock (shallow market depth)
-protocol
-risky investment vehicle
-daytrading tool
-information network (you can put info into the blockchain)
-pyramid system (first comers making money out of newcomers)
-could be used for voting
-excellent criminal usage for quick, hard to catch payments (best for black mail type of crimes)
-public ledger (quasi anonymous)
-a crazy exercise in human gullibility and mass psychology
-constant provider of incredible entertainment for thinking type of people
-etc.etc.
But basicly it is just a bunch of numbers on your computer or on a piece of paper or even on a physical coin.
Yes it can be both backed up but also can be one instance (like the physical bitcoins) although you have to trust the maker, that he didn't write down the numbers more than once...
You can find plenty of more detailed explanations and FAQs online, but if you want to read critical views of bitcoin (and you should) I advise to read reddit.com/r/Buttcoin
You see cult believers don't like to promote negative views and criticism of their system....