I've noted here before my impression that the mainstream "contemporary (post-/ex-/pseudo-/semi-)classical" has gradually become — for better and/or worse — a repertoire music,* a status that in the 20th century was largely associated with genre musics. With this development, we can reckon with more examples of the current Osvaldo Golijov sharing/borrowing/stealing controversy. (This thread at Sequenza 21 is keeping up with the story.)
Now, one of the necessary conditions of the formation of a musical repertoire is that numerous musical elements will be shared. These shared elements can be more abstract or structural in character — certain preferred forms or ensembles, for example — or they can be more immediately material — scales and modes, figures, riffs, sequences, progressions, tunes, scoring patterns, rhythmic tricks, even a characteristic dynamic profile (see Mannheim School).
The identity of an individual work in a repertoire with considerable sharing is often a matter of degree, and the individual stylistic preferences of a composer — whether as an expression of individual personality or of a shared, received aesthetic, or of a market identity — can be heard as value settings for the nature and degree of sharing in each category of material. However, when a composer borrows larger stretches of material, and particularly stretches which are recognizable as products of the imagination and labor of other musicians, then there is a body of terms with which one can describe the work: arrangements, variations, parodies, pastiche, citations etc.. It's even possible to sell wholesale appropriations as conceptual works.
In all of these cases going beyond the everyday commerce in shared materials which is standard practice in any repertoire, the composer is obliged to acknowledge his or her borrowings. Now it may well be that Mr Golijov, under the pressure of a large backlog of commissions, made an arrangement of some sort with his alleged source for the commission that has come under scrutiny. It could be, for example, a form of work-for-hire, a practice with some substantial tradition in all of the arts, in which case, the commissioners may have some concerns as to whether the work delivered was the "authentic" Golijov they asked (and paid) for. I suspect, however, that to whatever degree material was borrowed, the ultimate evaluation is going to be highly subjective in character, and it will be very interesting to hear how the rights organizations and Mr Golijov's academic institutions handle this, as they have their own interests and standards, obtained over long experience with the legal and moral niceties, for the correct identification of authorship.
For all of the attractions and advantages of working in a repertoire, priority and attributions are inevitable concerns for a composer.
* The avant-garde and radical-experimental impulses being, by nature —and also, for better and/or worse —, counter-repertorial.