Since this site frequently deals with larger statistical concepts, I frequently find that my explanations to users cover a lot of the same ground. For example, when I am answering a question on cross-validation, the beginning of my answer is always unique to the particular question, but at the end when I try to tie the particular example to the larger concept, I find myself saying similar things. I tell them the key to designing cross-validation is about thinking about how your new data you want to use the model on are different than your training data, and so you should try to split of your data for cross-validation so the cross-validation folds will mirror those novel aspects.
It seems like it might be more efficient to take the time to write out a few sentences illustrating this concept as eloquently as possible, and then copy and paste them at the end of the unique part of the answers. This would avoid rewriting it each time and have the quality of the explanation vary depending on other factors such as how much time I has to read over my answer. However, while the larger Stack Exchange community seems ok with copy and paste answers, Cross-Validated seems to frown on it.
So where would these partial copy and pastes fall on that scale? Are they ok because most of the answer is unique, or not because we only want unique content? I might consider linking to a previous answer with the repeated part, but usually the part I want to reference is just a few sentences at the end of an otherwise unrelated answer.