If you buy a cat and it gives birth, you have two, even though you only bought one. So what? There is no halacha prohibiting owning two of something that you only bought one of.
And if you buy a watch and disassemble it and locate and purchase all the third-party parts the watchmaker manafactured it with, and put a second exactly duplicated watch together on your own, you have deprived the original watchmaker the revenue of had you purchased it from him, that was his “intellectual property” in knowing what parts to use where. Have you stolen from him? Why was he entitled to have you buy it from him?
And perhaps you don’t believe the argument, that you wouldn’t have otherwise purchased it, can safely be used, but others disagree with you. Though, you are correct that one must be honest with himself about the truth of that.