The importance of DRY is proportional to what could go wrong if the repeated code has an issue. For instance, if the code is performing some vital calculation

The importance of DRY is proportional to what could go wrong if the repeated code has an issue. For instance, if the code is performing some vital calculation or is part of your security architecture, it had better not be repeated anywhere because somebody will need to patch it later and will need to guarantee that the update has been applied consistently. Sometimes, it's just clearer to rewrite something yourself. For instance, just because you can express just about anything in terms of algorithms in the C++ standard library, you should instead write the simpler stuff by hand. (A great example is this silly idea of “copying to an ostream iterator” just to print out some data; I don’t care if that combination of standard functions happens to produce the desired result, because the code is painful to look at!) It's also helpful to using aliasing (e.g. C++ reference variables) to make similar code look as similar as possible; i.e. rather than have two similar blocks using entirely different variable names throughout, declare references at the top to give them the same names so that the similarities are obvious. This also makes it easier to later pull common parts into functions if desired.