Duplication is probably the #1 enemy of clean code. Its presence make the code harder to maintain, harder to evolve, more bug prone, harder to test and probably eat babies every morning. If almost any developer will agree on that fact, me can also notice that almost every codebase is crippled with repetitions in the code. How could we explain that ?
On many Internet websites, you’ll come across article explaining you HTML and PHP. But none of them teach you that the PHP tag should be considered as an HTML element, not as a PHP one. Worse, many example you’ll find does this mistake again and again. This article is PHP centered, but apply to other technologies like JSP or most templates systems, where code is mixed with logic.