Why is this an issue?

A chain of if/else if 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.

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.

Noncompliant code example

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

Compliant solution

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

Resources

Documentation