Am I missing something?
Both work for me with 4.53...
Ran each code twice. First time got "No" and the directory is created. Second time got "Yes".
With each of the scripts.
Code:

Break On
$dest = "\\" + @WKSTA + "\c$\script\blablabla"
If NOT Exist($dest)
 ?"No"
 MD $dest
Else
 ?"Yes"
EndIf
? "Error " + @ERROR + ": " + @SERROR
? "KiX Version: " + @KIX

Code:

Break On
$dest = "\\" + @WKSTA + "\c$$\script\blablabla"
If NOT Exist($dest)
 ?"No"
 MD $dest
Else
 ?"Yes"
EndIf
? "Error " + @ERROR + ": " + @SERROR
? "KiX Version: " + @KIX