Duplication

"Every time you see duplication in the code, it represents a missed opportunity for abstraction." Duplication should be turned into another function or another class. Additionally, switch cases and if-else chains can represent similar ideas which should be replaced with polymorphism (slightly different classes inheriting from the same parent type)"

void a() {
    doThing1;
    doThing2;
    anotherThing1;
    anotherThing2;
}
void b() {
    doThing3;
    doThing4;
    anotherThing1;
    anotherThing2;
}

Could instead be

void a() {
    doThing1;
    doThing2;
    doAnotherThing();
}
void b() {
    doThing3;
    doThing4;
    doAnotherThing();
}
void doAnotherThing() { 
    anotherThing1;
    anotherThing2;
}

In terms of switch cases/if-else:

void internalClassFunction() {
    switch (internalClassCondition)
    {
        case A:
            doThingForA()
        case B:
            doThingForB()
        case C:
            doThingForC()
    }
}

Could instead be:

class Thing {
    virtual void internalClassFunction() = 0;
}
...
class ThingTypeA {
    void internalClassFunction() {
        doThingForA();
    }
}
class ThingTypeB {
    void internalClassFunction() {
        doThingForB();
    }
}
class ThingTypeC {
    void internalClassFunction() {
        doThingForC();
    }
}