Any valid reason for code duplication?

The valid reason I can think of: If the code gets alot more complex to avoid the duplication. Basically that's the place when you do almost the same in several methods - but just not quite the same. Of course - you can then refactor and add special parameters including pointers to different members that have to be modified. But the new, refactored method may get too complicated.

Example (pseudocode):

procedure setPropertyStart(adress, mode, value)
begin
  d:=getObject(adress)
  case mode do
  begin
    single: 
       d.setStart(SingleMode, value);
    delta:
       //do some calculations
       d.setStart(DeltaSingle, calculatedValue);
   ...
end;

procedure setPropertyStop(adress, mode, value)
begin
  d:=getObject(adress)
  case mode do
  begin
    single: 
       d.setStop(SingleMode, value);
    delta:
       //do some calculations
       d.setStop(DeltaSingle, calculatedValue);
   ...
end;

You could refactor out the method call (setXXX) somehow - but depending on the language it could be difficult (especially with inheritance). It is code duplication since most of the body is the same for each property, but it can be hard to refactor out the common parts.

In short - if the refactored method is factors more complicated, I'd go with code duplication although it is "evil" (and will stay evil).

in my humble opinion there's no place for code duplication. have a look, for example, at this wikipedia article

or, let's refer to Larry Wall's citation:

"We will encourage you to develop the three great virtues of a programmer: laziness, impatience, and hubris."

it is pretty clear that code duplication has nothing to do with "laziness". haha;)