I think mathematicians and scientists probably understand this best, as even though basing work off of (already discovered) mathematical and scientific principles is essential to making new discoveries, there have been many famous incidences in which two scientists or mathematicians discover a similar or the same concept, and one gets credit, even if it was discovered at around the same time (Newton and Leibniz discovering calculus, Darwin and Wallace discovering evolution, etc.). This usually happens because the circumstances of the time are right so that even if those people hadn't made the discovery, someone was bound to anyway.
This also applies to inventors. Though patents are granted to those who invented something first in the case of two very similar inventions submitted at the same time, and patents prevent anyone from gaining credit for the inventions they protect, there have been people who have invented and even received patents for an innovation which someone else later modifies and makes a lot of money off of. Sadly, whoever can sell the product better can end up getting the credit for something that wasn't originally theirs.
Plagiarism happens a lot in the world of scientific and mathematical research. In fact, there has been so much plagiarism of research that hearing about an idea and later believing that it is yours has been recognized as a psychological phenomenon by some people, therefore excusing those who copy other's ideas. Even when it wasn't plagiarism, many historical discoveries were strongly based off of previous ones (as in the case of Lobachevsky basing his system of non-Euclidean geometry off of Gauss', inspiring the hilarious Tom Lehrer song "Lobachevsky"). This can make it difficult for credit to be awarded to those who deserved it.
As for historians and philosophers, I can understand having credit problems in historical research, but considering that philosophy is based on questions that never can really be factually answered, the only credit given to philosophers is on whether they stated and presented a philosophical concept. It's hard to invent a philosophical concept, in my opinion. You can state and provide evidence for why you think it's a sound way to see the world, but does that mean you thought of it first? For example, George Berkeley was perhaps one of the leading thinkers in idealism, but the immaterialistic concept goes back to Plato, and the Ancient Greeks (not to mention probably other, uncredited civilizations) were probably pondering whether their world was real before Plato. Does that mean Berkeley and Plato shouldn't get credit for their accomplishments? No, it just means that how we define "credit" in terms of philosophy should be different than the definition of "credit" in other intellectual fields.
This is just bouncing around in my head as an interesting topic to think about, that I have a few thoughts on.
Recently in a vote chess game player A posted a sequence of 3 moves. Another player B pointed out that this sequence had been given already in the Archives by another vote chess player (C). I think the intention was to correctly attribute precedence, thus awarding C some credit for the intellectual achievement of having stated the sequence in the record.
This kind of process is an essential feature of literature and scholarly work. I don't believe I ever studied it intentionally. I recall I first encountered it in a history course in about 7th grade. My parent had argued with the teacher and she may not have taught us well, but I was to learn different citation styles.
It's something of an artificial measure, as the experience and result of discovering and sharing a thing is somewhat independent of what has been stated previously in some less accessible form.
Anyway, it definitely seems like an interesting area for discussion; does anybody want to write more about it? What are some good sources on this topic? What kind of academics understand it best, historians or philosophers or scientists?