Solution: A diamond of classes

Here is a possible solution to the task.

Code

class A {
}

class B is A {
}

class C is A {
}

class D is B is C {
}

say D.^mro.map(*.^name);

🦋 You can find the source code in the file diamond-mro.raku.

(D B C A Any Mu)

D inherits from two parents at once — class D is B is C lists each of them with its own is. This is multiple inheritance, and A, B, C, D form a diamond: two paths from D up to the shared ancestor A.
.^mro flattens that diamond into a single, linear search order. D comes first, then its parents B and C in the order they were written, then the shared A, and finally Any and Mu.
Even though both B and C lead to A, the type A appears once, and only after both of them. That is the point of a method resolution order: every type is visited exactly once, and a parent never comes before a child that inherits from it — so a method defined in B is always found before the one it would override in A.