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();
}