1. That is not well-defined. What does double-spending even mean for Bitcoin? Previous security definitions for digital cash define double spending in terms of a central bank that issues the money.
2. Even with the vague idea of what double-spending means for Bitcoin, your statement is false under the definition of "hard" that cryptographers typically use. There is a known polynomial-time algorithm for double spending Bitcoin that was described in the original Bitcoin whitepaper. That attack is practical and there are already concerns about one "mining pool" that has almost enough computational power to pull it off (and if they are willing to accept a small probability of failure, they already have that much power).
Yawn. Fraud is easy, no matter how you define it, stealing a few bitcoins on the other hand merely takes a finite amount of resources.
You're essentially a troll. What you say is true but double spending a few coins (your maximum return is 2x) would take hundreds of thousands of dollars and a lot of luck. Duplicating paper money requires an inkjet printer with good registration and the common sense to separate the eruion constellations into separate layers. It's not rocket surgery, and once the source file is made it can be replicated at any copy shop. Its vanishingly unlikely you haven't accepted a counterfeit bill, and its vanishingly unlikely a bitcoin could be double-spent beyond seven blocks.
Give it up. Bitcoins may not be a sound investment but they're far more secure than cash for both parties.
1. That is not well-defined. What does double-spending even mean for Bitcoin? Previous security definitions for digital cash define double spending in terms of a central bank that issues the money.
2. Even with the vague idea of what double-spending means for Bitcoin, your statement is false under the definition of "hard" that cryptographers typically use. There is a known polynomial-time algorithm for double spending Bitcoin that was described in the original Bitcoin whitepaper. That attack is practical and there are already concerns about one "mining pool" that has almost enough computational power to pull it off (and if they are willing to accept a small probability of failure, they already have that much power).