hcl/json: catch and reject duplicate attrs in JustAttributes

Previously this was handled in the parser, but the parser now permits
multiple properties with the same name and so we must handle this at the
decoder level instead.

hcl/json/structure.go

@@ -192,6 +192,17 @@ func (b *body) JustAttributes() (hcl.Attributes, hcl.Diagnostics) {
 		if _, hidden := b.hiddenAttrs[name]; hidden {
 			continue
 		}
+
+		if existing, exists := attrs[name]; exists {
+			diags = append(diags, &hcl.Diagnostic{
+				Severity: hcl.DiagError,
+				Summary:  "Duplicate attribute definition",
+				Detail:   fmt.Sprintf("The attribute %q was already defined at %s.", name, existing.Range),
+				Subject:  &jsonAttr.NameRange,
+			})
+			continue
+		}
+
 		attrs[name] = &hcl.Attribute{
 			Name:      name,
 			Expr:      &expression{src: jsonAttr.Value},

hcl/json/structure_test.go

@@ -1096,12 +1096,14 @@ func TestJustAttributes(t *testing.T) {
 	// We test most of the functionality already in TestBodyPartialContent, so
 	// this test focuses on the handling of extraneous attributes.
 	tests := []struct {
-		src  string
-		want hcl.Attributes
+		src       string
+		want      hcl.Attributes
+		diagCount int
 	}{
 		{
 			`{}`,
 			map[string]*hcl.Attribute{},
+			0,
 		},
 		{
 			`{"foo": true}`,
@@ -1130,10 +1132,41 @@ func TestJustAttributes(t *testing.T) {
 					},
 				},
 			},
+			0,
 		},
 		{
 			`{"//": "comment that should be ignored"}`,
 			map[string]*hcl.Attribute{},
+			0,
+		},
+		{
+			`{"foo": true, "foo": true}`,
+			map[string]*hcl.Attribute{
+				"foo": {
+					Name: "foo",
+					Expr: &expression{
+						src: &booleanVal{
+							Value: true,
+							SrcRange: hcl.Range{
+								Filename: "test.json",
+								Start:    hcl.Pos{Byte: 8, Line: 1, Column: 9},
+								End:      hcl.Pos{Byte: 12, Line: 1, Column: 13},
+							},
+						},
+					},
+					Range: hcl.Range{
+						Filename: "test.json",
+						Start:    hcl.Pos{Byte: 1, Line: 1, Column: 2},
+						End:      hcl.Pos{Byte: 12, Line: 1, Column: 13},
+					},
+					NameRange: hcl.Range{
+						Filename: "test.json",
+						Start:    hcl.Pos{Byte: 1, Line: 1, Column: 2},
+						End:      hcl.Pos{Byte: 6, Line: 1, Column: 7},
+					},
+				},
+			},
+			1, // attribute foo was already defined
 		},
 	}
 
@@ -1144,8 +1177,8 @@ func TestJustAttributes(t *testing.T) {
 				t.Fatalf("Parse produced diagnostics: %s", diags)
 			}
 			got, diags := file.Body.JustAttributes()
-			if len(diags) != 0 {
-				t.Errorf("Wrong number of diagnostics %d; want %d", len(diags), 0)
+			if len(diags) != test.diagCount {
+				t.Errorf("Wrong number of diagnostics %d; want %d", len(diags), test.diagCount)
 				for _, diag := range diags {
 					t.Logf(" - %s", diag)
 				}