One can approach a moral position on plagiarism in the Catholic morality from one of two ways. One can view it as an act of stealing in that plagiarism is taking something which does not belong to you (credit for a specific work) and claiming it as your own in order to receive some good, or one can view it as an act of theft.
According to the Catechism of the Catholic Church paragraph 2508, lying is
Lying consists in saying what is false with the intention of deceiving one's neighbor
Certainly this is what one is doing in plagiarism. Therefore according to paragraph 2485
By its very nature, lying is to be condemned. It is a profanation of speech, whereas the purpose of speech is to communicate known truth to others.
Paragraph 1753 also defines lying as instrinsically disordered and as such it is a mortal sin.
When viewed from the perspective as a lie, plagiarism can not be contanensed by Catholic morality.
To look at it as stealing we see that paragraph 2408 defines theft as
The seventh commandment forbids theft, that is, usurping another's property against the reasonable will of the owner
Therefore if we consider the definition of "property" to include what is universally considered "intellectual property", then we would have to accept that plagiarism is theft. The Seventh Commandment forbids theft of this nature and to knowingly break a Commandment is a mortal sin. Again, it can not be sanctioned within Catholic morality.
The only question left is whether one would intervene when they sees someone else committing plagiarism. If we can answer that we would intervene if we saw someone stealing or lying to cheat someone out of what is rightfully theirs, then we must agree that the Catholic is required not only to avoid plagiarism themselves but must also intervene when they see it happening with other.