Hello,
 
I am working with AUTOCAD 2008 LT,  and there are two dimention lines not showing in layout , but they could be plotted out.  Also I can see them in model space which have the exactly same properties as the other dimension on the same layer.
 
Who can tell how it happened and how to fix this problem because I do not want to double these dimension in the drawing.
 
 
thanks
 
 

The layers visibilty in model and layout can be diffrent, check it please.

if you doble click the viewport of the layout, you can see if there is a frezze layer in it. perhaps you have some layers frezze or off in the layout and not in the model.

This type of "strange behavior - call ghostbusters" is often caused by an invalid combination of layer off/freeze for the object+dimension+defpoints layers or by using a "forged" DWG file from a non-Autodesk CAD application.