Having two cases in the same switch statement or branches in the same if structure with the same implementation is at best duplicate code, and at worst a coding error. If the same logic is truly needed for both instances, then in an if structure they should be combined, or for a switch, one should fall through to the other.

Noncompliant Code Example

switch (i)
{
  case 1:
    DoFirst();
    DoSomething();
    break;
  case 2:
    DoSomethingDifferent();
    break;
  case 3:  // Noncompliant; duplicates case 1's implementation
    DoFirst();
    DoSomething();
    break;
  default:
    DoTheRest();
}

if (a >= 0 && a < 10)
{
  DoFirst();
  DoTheThing();
}
else if (a >= 10 && a < 20)
{
  DoTheOtherThing();
}
else if (a >= 20 && a < 50)   // Noncompliant; duplicates first condition
{
  DoFirst();
  DoTheThing();
}

Exceptions

Blocks in an if chain that contain a single line of code are ignored, as are blocks in a switch statement that contain a single line of code with or without a following break.

if (a >= 0 && a < 10)
{
  DoTheThing();
}
else if (a >= 10 && a < 20)
{
  DoTheOtherThing();
}
else if (a >= 20 && a < 50)    //no issue, usually this is done on purpose to increase the readability
{
  DoTheThing();
}

But this exception does not apply to if chains without else-s, or to switch-es without default clauses when all branches have the same single line of code. In case of if chains with else-s, or of switch-es with default clauses, rule {rule:csharpsquid:S3923} raises a bug.

if(a == 1)
{
  doSomething();  //Noncompliant, this might have been done on purpose but probably not
}
else if (a == 2)
{
  doSomething();
}