Why is this an issue?

Having two cases in a switch statement or two branches in an if chain with the same implementation is at best duplicate code, and at worst a coding error.

switch (i) {
  case 1:
    doFirstThing();
    doSomething();
    break;
  case 2:
    doSomethingDifferent();
    break;
  case 3:  // Noncompliant; duplicates case 1's implementation
    doFirstThing();
    doSomething();
    break;
  default:
    doTheRest();
}

if (a >= 0 && a < 10) {
  doFirstThing();
  doTheThing();
}
else if (a >= 10 && a < 20) {
  doTheOtherThing();
}
else if (a >= 20 && a < 50) {
  doFirstThing();
  doTheThing();  // Noncompliant; duplicates first condition
}
else {
  doTheRest();
}

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

• in an if chain they should be combined

if ((a >= 0 && a < 10) || (a >= 20 && a < 50)) { // Compliant
  doFirstThing();
  doTheThing();
}
else if (a >= 10 && a < 20) {
  doTheOtherThing();
}
else {
  doTheRest();
}

• for a switch, one should fall through to the other.

switch (i) {
  case 1:
  case 3: // Compliant
    doFirstThing();
    doSomething();
    break;
  case 2:
    doSomethingDifferent();
    break;
  default:
    doTheRest();
}

When all blocks are identical, either this rule will trigger if there is no default clause or rule {rule:flex:S3923} will raise if there is a default clause.

Exceptions

Unless all blocks are identical, blocks in an if chain that contain a single line of code are ignored. The same applies to blocks in a switch statement that contains a single line of code with or without a following break.

if (a == 1) {
  doSomething();  //no issue, usually this is done on purpose to increase the readability
} else if (a == 2) {
  doSomethingElse();
} else {
  doSomething();
}

Resources

Related rules

• {rule:flex:S3923} - All branches in a conditional structure should not have exactly the same implementation