Oh, I found that under some conditions HitTest needs coordinates multiplied by 15. (AFAIK to convert from pixels to VB units but I'm not sure).
Very weird thing.

Please try this modified code:

LOCAL laMice[4]
IF AMOUSEOBJ(laMice) = 4 THEN
  IF NOT ISNULL(This.HitTest(15*laMice[3], 15*laMice[4])) THEN
    * Doubleclick action
  ENDIF
ENDIF

Hope this helps.