diff --git a/Assets/VRM/Runtime/SpringBone/Logic/SpringBoneSystem.cs b/Assets/VRM/Runtime/SpringBone/Logic/SpringBoneSystem.cs
index a44091bf1..4942a4c11 100644
--- a/Assets/VRM/Runtime/SpringBone/Logic/SpringBoneSystem.cs
+++ b/Assets/VRM/Runtime/SpringBone/Logic/SpringBoneSystem.cs
@@ -38,7 +38,15 @@ namespace VRM.SpringBone
 
             for (int i = 0; i < m_joints.Count; ++i)
             {
-                m_jointIndexMap.Add(m_joints[i].Item1, i);
+                // #2617
+                // Keys may not be unique
+                m_jointIndexMap[m_joints[i].Item1] = i;
+
+                // this index refer m_joints
+                // if not unique,
+                // m_joints[] = { (a, a_state), (b, b_state), (a, a_state) ... };
+                // m_joints[0] and m_joints[2] is same.
+                // The above code is ok
             }
         }
 
diff --git a/Assets/VRM/Tests/VRMSpringBoneTests.cs b/Assets/VRM/Tests/VRMSpringBoneTests.cs
new file mode 100644
index 000000000..c9182d616
--- /dev/null
+++ b/Assets/VRM/Tests/VRMSpringBoneTests.cs
@@ -0,0 +1,31 @@
+using System.IO;
+using System.Linq;
+using NUnit.Framework;
+using UnityEngine;
+
+namespace VRM
+{
+    public class VRMSpringBoneTests
+    {
+        static string AliciaPath
+        {
+            get
+            {
+                return Path.GetFullPath(Application.dataPath + "/../Tests/Models/Alicia_vrm-0.51/AliciaSolid_vrm-0.51.vrm")
+                    .Replace("\\", "/");
+            }
+        }
+
+        [Test]
+        public void DuplicatedJoints()
+        {
+            using var loaded = TestVrm0X.LoadPathAsBuiltInRP(AliciaPath);
+            var sb = loaded.GetComponentInChildren<VRMSpringBone>();
+
+            // make duplicate #2617
+            sb.RootBones.Add(sb.RootBones[0].GetChild(0));
+
+            Assert.DoesNotThrow(() => sb.Setup());
+        }
+    }
+}
\ No newline at end of file
diff --git a/Assets/VRM/Tests/VRMSpringBoneTests.cs.meta b/Assets/VRM/Tests/VRMSpringBoneTests.cs.meta
new file mode 100644
index 000000000..715be0a3b
--- /dev/null
+++ b/Assets/VRM/Tests/VRMSpringBoneTests.cs.meta
@@ -0,0 +1,11 @@
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