Having two Cases in the same Select 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 Select, one should fall through to the other.

Noncompliant Code Example

Select i
  Case 1
    DoFirst()
    DoSomething()
  Case 2
    DoSomethingDifferent()
  Case 3  ' Noncompliant; duplicates case 1's implementation
    DoFirst()
    DoSomething()
  Case Else:
    DoTheRest()
End Select

If a >= 0 AndAlso a < 10 Then
  DoFirst()
  DoTheThing()
ElseIf a >= 10 AndAlso a < 20 Then
  DoTheOtherThing()
ElseIf a >= 20 AndAlso a < 50   ' Noncompliant; duplicates first condition
  DoFirst()
  DoTheThing()
Else
  DoTheRest();
End If

Exceptions

Blocks in an If chain or Case clause that contain a single line of code are ignored.

If a >= 0 AndAlso a < 10 Then
  DoTheThing()
ElseIf a >= 10 AndAlso a < 20 Then
  DoTheOtherThing()
ElseIf a >= 20 AndAlso a < 50   ' no issue, usually this is done on purpose to increase the readability
  DoTheThing()
End If

But this exception does not apply to If chains without Else-s, or to Select-s without Case Else clauses when all branches have the same single line of code. In case of If chains with Else-s, or of Select-es with Case Else clauses, rule {rule:vbnet:S3923} raises a bug.

If a >= 0 AndAlso a < 10 Then
  DoTheThing()
ElseIf a >= 10 AndAlso a < 20 Then
  DoTheOtherThing()  ' Noncompliant, this might have been done on purpose but probably not
End If