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Given a set of triples of a base set $S$, find a subset of triples such that each element in $S$ appears exactly in one triple. This problem is NP-complete by reduction from NP-complete problem 3D Matching.

I'm interested in the complexity of related problem. What is the complexity of finding a subset of triples such that each element in $S$ appears exactly in two triples?

Has this problem been studied in the literature? I'd greatly appreciate pointing me to references.

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The problem seems to admit a fairly straightforward reduction from 3D matching. – Colin McQuillan Apr 24 '11 at 17:11
    
Which reduction do you have in mind for NP-hardness? – Mohammad Al-Turkistany Apr 24 '11 at 18:25

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