The relation closure for wordnet synsets is supposed to prevent duplicates in the output. However, the duplicates check fails to detect some repetitions, which occur when there are multiple paths to a given synset. In the following output for ex., the branch going from 'taxonomic_group.n.01' to 'entity.n.01' appears twice, because it is reachable by two different paths:

from nltk.corpus import wordnet as wn
ss=wn.synset("calamagrostis.n.01")
print(list(ss.closure(lambda s: s.hypernyms())))

[Synset('gramineae.n.01'), Synset('monocot_genus.n.01'), Synset('monocot_family.n.01'), Synset('genus.n.02'), Synset('family.n.06'), Synset('taxonomic_group.n.01'), Synset('taxonomic_group.n.01'), Synset('biological_group.n.01'), Synset('biological_group.n.01'), Synset('group.n.01'), Synset('group.n.01'), Synset('abstraction.n.06'), Synset('abstraction.n.06'), Synset('entity.n.01'), Synset('entity.n.01')]

Produce an SVG image to illustrate this graph:

from nltk.parse.dependencygraph import dot2img
print(dot2img(wn.digraph([ss])))