What if I import the same module twice?
That's not a problem. When three modules all import Module 'A', Angular evaluates Module 'A' once, the first time it encounters it, and doesn't do so again.
That's true at whatever level A appears in a hierarchy of imported modules. When Module 'B' imports Module 'A', Module 'C' imports 'B', and Module 'D' imports [C, B, A], then 'D' triggers the evaluation of 'C', which triggers the evaluation of 'B', which evaluates 'A'. When Angular gets to the 'B' and 'A' in 'D', they're already cached and ready to go.
Angular doesn't like modules with circular references, so don't let Module 'A' import Module 'B', which imports Module 'A'.