Alt container duplicate error

I have submitted an Alt container which has a duplicate 
"http://recshop.fake/cd#CD1" item, but the parser didn't catch it and 
got it as valid:

<rdf:Alt>
<rdf:_1 rdf:resource="*http://recshop.fake/cd#CD1*" />
<rdf:_2 rdf:resource="http://recshop.fake/cd#CD2" />
<rdf:_3 rdf:resource="*http://recshop.fake/cd#CD1*" />
</rdf:Alt>

So, can Alt containers have duplicates?
Thanks.