It's not possible to show "same" field twice in a single form view. Possible ways can be,

1. Don't show that field in notebook pages. Just show it to top of Form view. Just like in Sale order form, Quotation No. field.

2. Make an another field Y ( kind of field.function ) which will have value of that X.  After that you can use that field Y to another page of notebook. ( Here I would like to mention that this way is not good )

Field "Y" definition in py file can be,

 @api.one
 @api.depends( 'X' )
 def get_X_name( self ):
     if self.X:    
         self.Y = self.X
 Y = fields.Char( 'AB', compute=get_X_name , store=True )

Hope this helps,

Try to create a function field and store to it the value of the original field and then put the original field in the first page and the function field in the other page