fix: don't let case-insensitive fallback reuse an exactly-matched key

mapstructure.go

@@ -1681,6 +1681,14 @@ func (d *Decoder) decodeStructFromMap(name string, dataVal, val reflect.Value) e
 					continue
 				}
 
+				// Skip keys already consumed by another field's exact
+				// match, so a case-insensitive fallback can't claim the
+				// same key a second time (e.g. fields `Name` and `NAME`
+				// both matching the key "name").
+				if _, unused := dataValKeysUnused[dataValKey.Interface()]; !unused {
+					continue
+				}
+
 				if d.config.MatchName(mK, fieldName) {
 					rawMapKey = dataValKey
 					rawMapVal = dataVal.MapIndex(dataValKey)

mapstructure_test.go

@@ -4673,3 +4673,25 @@ func TestUnmarshaler_StructToMap(t *testing.T) {
 		t.Errorf("expected Age 30, got %v", result["Age"])
 	}
 }
+
+func TestDecode_caseInsensitiveDuplicateKey(t *testing.T) {
+	// A case-insensitive fallback must not claim a key that another field
+	// already matched exactly: decoding {"name": ...} into a struct with both
+	// `name` and `NAME` fields should populate only the exact match.
+	type Result struct {
+		Name string `mapstructure:"name"`
+		NAME string `mapstructure:"NAME"`
+	}
+
+	var result Result
+	if err := Decode(map[string]interface{}{"name": "lower"}, &result); err != nil {
+		t.Fatalf("got an err: %s", err)
+	}
+
+	if result.Name != "lower" {
+		t.Errorf("Name should be 'lower', got: %#v", result.Name)
+	}
+	if result.NAME != "" {
+		t.Errorf("NAME should be empty, got: %#v", result.NAME)
+	}
+}