Intertextuality means the interrelationship between texts, especially works of literature; the way that similar or related textsinfluence, reflect, or differ from each other: the intertextuality between two novels with the same setting. [dictionary.reference.com]
Youtube video to help you understand the concept.
Even the great speech 'I have a dream' uses a lot of rhetorical devices in order to allude other concepts especially from Bible, Black people's song, etc. It is to say that the idea from speech is not original to Martin Luther King jr. In near future, I am going to research about intertextuality in 'I have a dream'
According to this concept, we can say that every idea we have is not original but related to some other concept whether we intended or not. This leads to that we are very influenced by others than we know.