Detect and warn on duplicate map keys

gohcl/decode.go

@@ -240,16 +240,27 @@ func decodeBodyToMap(body hcl.Body, ctx *hcl.EvalContext, v reflect.Value) hcl.D
 	mv := reflect.MakeMap(v.Type())
 
 	for k, attr := range attrs {
+		key := reflect.ValueOf(k)
+		var value reflect.Value
 		switch {
 		case attrType.AssignableTo(v.Type().Elem()):
-			mv.SetMapIndex(reflect.ValueOf(k), reflect.ValueOf(attr))
+			value = reflect.ValueOf(attr)
 		case exprType.AssignableTo(v.Type().Elem()):
-			mv.SetMapIndex(reflect.ValueOf(k), reflect.ValueOf(attr.Expr))
+			value = reflect.ValueOf(attr.Expr)
 		default:
 			ev := reflect.New(v.Type().Elem())
 			diags = append(diags, DecodeExpression(attr.Expr, ctx, ev.Interface())...)
-			mv.SetMapIndex(reflect.ValueOf(k), ev.Elem())
+			value = ev.Elem()
 		}
+		if mv.MapIndex(key).IsValid() {
+			diags = append(diags, &hcl.Diagnostic{
+				Severity: hcl.DiagWarning,
+				Summary:  "Duplicate map key",
+				Detail:   fmt.Sprintf("Key '%s' was defined twice in this map.", key),
+				// Subject:  ???
+			})
+		}
+		mv.SetMapIndex(key, value)
 	}
 
 	v.Set(mv)

gohcl/decode_test.go

@@ -137,6 +137,35 @@ func TestDecodeBody(t *testing.T) {
 			},
 			0,
 		},
+		// Duplicate key
+		{
+			map[string]interface{}{
+				"name": "Ermintrude",
+			},
+			makeInstantiateType(withNameExpression{}),
+			func(v interface{}) bool {
+				if v == nil {
+					return false
+				}
+
+				wne, valid := v.(withNameExpression)
+				if !valid {
+					return false
+				}
+
+				if wne.Name == nil {
+					return false
+				}
+
+				nameVal, _ := wne.Name.Value(nil)
+				if !nameVal.Equals(cty.StringVal("Ermintrude")).True() {
+					return false
+				}
+
+				return true
+			},
+			0,
+		},
 		{
 			map[string]interface{}{
 				"name": "Ermintrude",