Add nested duplicate merge key test cases

Author: aaronbronow

tests/legacy_tests/data/construct-merge.code

@@ -9,4 +9,7 @@
     { "x": 1, "y": 2, "r": 10, "label": "center/big" },
     { "x": 1, "y": 2, "r": 10, "label": "center/big" },
     { "x": 1, "y": 2, "r": 10, "label": "center/big" },
+    { "x": 1, "y": 2, "r": 10, "label": "center/big" },
+    { "x": 1, "y": 2, "r": 10, "label": "center/big" },
+    { "x": 1, "y": 2, "r": 10, "label": "center/big" },
 ]

tests/legacy_tests/data/construct-merge.data

@@ -36,3 +36,12 @@
   << : *CENTER
   r: 10
   label: center/big
+
+- # Nested duplicate merge level 0
+  &NESTED_L0 { x: 1, y: 2, r: 10, label: center/big }
+
+- # Nested duplicate merge level 1
+  &NESTED_L1 { <<: [ *NESTED_L0, *NESTED_L0 ] }
+
+- # Nested duplicate merge level 2
+  &NESTED_L2 { <<: [ *NESTED_L1, *NESTED_L1 ] }