Shadow is rendered twice in differents transparent planes

Hi,

I'm using AR Foundation and I try to put a transparent material in ARPlane that receives shadows. I'm using this shader:

---

 Shader "Custom/Shadows"
 {
     Properties
     {
         _ShadowIntensity("Shadow Intensity", Range(0, 1)) = 0.6
     }
 
 
         SubShader
     {
 
         Tags {"Queue" = "AlphaTest" }
 
         Pass
         {
             Tags {"LightMode" = "ForwardBase" }
             Cull Back
             Blend SrcAlpha OneMinusSrcAlpha
             CGPROGRAM
             #pragma vertex vert
             #pragma fragment frag
             #pragma multi_compile_fwdbase
 
             #include "UnityCG.cginc"
             #include "AutoLight.cginc"
             uniform float _ShadowIntensity;
 
             struct v2f
             {
                 float4 pos : SV_POSITION;
                 LIGHTING_COORDS(0,1)
             };
             v2f vert(appdata_base v)
             {
                 v2f o;
                 o.pos = UnityObjectToClipPos(v.vertex);
                 TRANSFER_VERTEX_TO_FRAGMENT(o);
 
                 return o;
             }
             fixed4 frag(v2f i) : COLOR
             {
                 float attenuation = LIGHT_ATTENUATION(i);
                 return fixed4(0,0,0,(1 - attenuation) * _ShadowIntensity);
             }
             ENDCG
         }
 
     }
         Fallback "VertexLit"
 }

---

It's work fine, but when I put 2 ARPlane in differents height, the shadow of an object higher then the top plane also is rendered in the bottom plane.

Is anyone have some idea of how to solve this?

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