Introducing similar topic in two papers
Topic C is seemingly a prerequisite to topics A &amp; B, therefore topic C must be introduced in papers that deal with topic A, B or both, to ensure that such papers are self-contained. That said, if topic C is standard, then there's no need for an in-depth introduction and a citation to an introduction will suffice.

It is unclear whether the introduction to topic C in the context of topic A will be the same as such an introduction in the context of topic B, because different aspects of topic C will be relevant. So, some differences between introductions are possibly when topic C is introduced in the context of topic A and when topic C is introduced in the context of topic B.