A chain of if-else-if or switch-case statements is evaluated from top to bottom. At most, only one branch will be executed: the first one with a condition that evaluates to true or that matches the discriminant of the switch.

Therefore, duplicating a condition automatically leads to dead code. Usually, this is due to a copy/paste error. At best, it’s simply dead code and at worst, it’s a bug that is likely to induce further bugs as the code is maintained, and obviously it could lead to unexpected behavior.

Note that this rule requires Node.js to be available during analysis.

Noncompliant Code Example

if (param == 1)
  openWindow();
else if (param == 2)
  closeWindow();
else if (param == 1)  // Noncompliant
  moveWindowToTheBackground();

switch (param) {
  case 1:
    openWindow();
    break;
  case 2:
    closeWindow();
    break;
  case 1: // Noncompliant
    moveWindowToTheBackground();
    break;
}

Compliant Solution

if (param == 1)
  openWindow();
else if (param == 2)
  closeWindow();
else if (param == 3)
  moveWindowToTheBackground();

switch (param) {
  case 1:
    openWindow();
    break;
  case 2:
    closeWindow();
    break;
  case 3:
    moveWindowToTheBackground();
    break;
}