If I single handedly write a piece of SW that does "X" and sell it to Yahoo. Can I then make a new site / app that dose "X"?
One example that comes to mind is Miller Puckette, who wrote Max, and then a few years later wrote its main open-source competitor, Pure Data. It seems quite possible to write a clone of your own software without literally writing the same thing, especially if it's been a few years and you have some new ideas about how to do it now. My guess is that doing so would most often be sunk, if it is, by software patents or explicit clauses in the sale terms, rather than copyright.
One example that comes to mind is Miller Puckette, who wrote Max, and then a few years later wrote its main open-source competitor, Pure Data. It seems quite possible to write a clone of your own software without literally writing the same thing, especially if it's been a few years and you have some new ideas about how to do it now. My guess is that doing so would most often be sunk, if it is, by software patents or explicit clauses in the sale terms, rather than copyright.