If the same logic is truly needed for both instances, then:

* in an `if` chain they should be combined

[source,apex,diff-id=1,diff-type=compliant]
----
if ((a >= 0 && a < 10) || (a >= 20 && a < 50)) {
  doFirstThing();
  doTheThing();
}
else if (a >= 10 && a < 20) {
  doTheOtherThing();
}
else {
  doTheRest(); 
}
----

* for a `switch`, the values should be put in the `when` values list.

[source,apex,diff-id=2,diff-type=compliant]
----
switch on i {
  when 1, 3 {
    doFirstThing();
    doSomething();
  }
  when 2 {
    doSomethingDifferent();
  }
  when else {
    doTheRest();
  }
}
----