Q. What is self-plagiarism and double-dipping -- in terms of the IB?
From the IBO Extended Essay Guide (2018):
“Double-dipping”: students must ensure that they are not using material submitted for any other assessment component as part of their EE submission—see the subject-specific guidance for more details.
Self-plagiarism is using or re-using work you have done before without acknowledging it.
If you do have previous work that you want to reference, you need to cite yourself.
For example, perhaps you submitted work for an Internal Assessment (IA) for IB Diploma history where you discussed the types of Marxism in Latin America. You later write your Extended Essay on Allende and Chilean Marxism, but want to summarize the types of Marxism in Latin America.
In your bibliography, you would include your IA in the alphabetical list of sources:
Brown, Nancy. "The Cuban Revolution." United World College of SE Asia, East campus. IB Diploma History internal assessment. 5 Jan. 2017.
You could then cite your IA as follows:
The types of Marxism in Latin America can be summarized as blah, blah, and blah (Brown).