Double interface

When I have a double interface, i.e. 2 interfaces at the same location, it seems that any interface can detect it target.
A simple example is given below for a double interface separating a cube at the mid-height under gravity. If a simple interface is created (switching the commented line and the line above), then it works.
What is the reason and how to make double interface works ?

model new
model large-strain off
model gravity 10

zone create brick
zone group "lower" range p-z 0 5
zone group "upper" range group "lower" not
zone face group "middle" internal range p-z 5
zone separate by-face rang group "middle"

zone interface "double" create by-face range group "middle"
; zone interface "double" create by-face range group "middle" group "upper"

zone cmodel assign elastic
zone prop dens 2e3 young 2e9 pois 0.2

zone interface node property stiff-normal 2e11 stiff-shear 2e11 

zone gridpoint fix vel-z range p-z 0


Your question is not clear. What is your problem?
Usually, interfaces are only assigned to one side and will work as intended. If for some reason you have a situation where multiple interfaces will be in contact, then maybe the troubleshooting section of the manual will help you:

Interface nodes can only register contact with one face at a time. This can cause problems in the areas of multiple intersecting interfaces (see Figure 3). Interface node A on the bottom surface will register contact with either target face b or face c. The order in which the interfaces are specified, or the effect of rounding off, determines this. This means that either gridpoint B or C will not register an appropriate contact and resisting force, causing arching at the interface connection. One solution is to declare two interfaces facing each other, each with half the stiffness and strength of the whole; another is to increase the number of interface nodes in that area. Either of these solutions will reduce the problem, but may not eliminate it.

Hi XP,
Try it if you still want a double interface.

zone interface “double1” create by-face range group “middle” group “upper”
zone interface “double2” create by-face range group “middle” group “lower”

Hi Markus,
It was a typo, I want to say neither of the 2 interfaces cannot detect the target. You can make a query on any interface node, the target is vacant.

Thanks Huy,
I know this workaround. However, if the 2 interfaces have the same name, for example just “double” instead of “double1” and “double2”, then it does not detect target again. In previous version, for example 5, or maybe 6 and 7, I did not observe this.

does this section works for 3DEC also or just for flac?

zone interface “double1” create by-face range group “middle” group “upper”
zone interface “double2” create by-face range group “middle” group “lower”

In 3DEC, all interface elements (contacts & subcontacts) are created and detected automatically.