Needing to mix and match
While it might seem like the creators of a given document should be able to keep their prefixes straight, there are cases where documents are assembled from multiple sources. In these cases - and in other cases where people are attached to prefixes - it's possible to assign the same prefix to different URIs by overriding the declaration.
You can redeclare a prefix or the default, and it will override previous declarations within the scope of the element. For example:
While overriding declarations can be necessary, it definitely inflicts a clarity cost on human readers, as well as a small stack-management cost for XML parsers. It also makes it difficult to process XML using text-processing tools.