Repetition also makes it harder to understand a system: not only do you have to read more, you also need to remember and compare repeating fragments that may be identical or just similar.

What makes it easier to understand a system is simplicity. I'd argue that DRY, deployed with a right strategic plan, usually does more to simplify things than does copy-paste.

But, as any tool, DRY is but a tool; to be useful it requires some skill.