Repeating a prefix, does not replace previous one

[roddolf]

When a query have a same prefix namespace repeated:

/* ... */
  .prefix( "ex", "https://example.com/ns#" )
  .prefix( "ex", "https://example.org/ns#" )

The query string returns both of them.
The expected string should just return the last set, i.e. https://example.org/ns#.