Guys, when I transfer/duplicate the same box to create a sliding door I don't get the empty line in the middle of duplicated box, why?

Hi everyone, I want to create a sliding door based on a floor plan. When I drag a box to the doorway to create a sliding door, I get the empty line in the middle of the box. When I duplicate and transfer the same box to the next doorway, I don’t get the same empty line. The only way is just to delete one of the boxes to get the empty line. Could you please let me know how to solve it, I mean how to get empty lines in both?