Create a solution for prefixes that are defined multiple times at prefix.cc

E.g. 'sem' is defined 2 times in the 'share/prefix.cc' file. It is not clear which definition is returned by the RDF::NS module (the first? the second? random?):

```
sem	http://semanticweb.cs.vu.nl/2009/11/sem/	20130802
sem	http://www.ontologydesignpatterns.org/cp/owl/semiotics.owl#	20160408
```
