diff --git a/Canon.Core/SemanticParser/CodeGeneratorVisitor.cs b/Canon.Core/SemanticParser/CodeGeneratorVisitor.cs
index 3e58f1c..0285373 100644
--- a/Canon.Core/SemanticParser/CodeGeneratorVisitor.cs
+++ b/Canon.Core/SemanticParser/CodeGeneratorVisitor.cs
@@ -31,7 +31,7 @@ public class CodeGeneratorVisitor(ICompilerLogger? logger = null) : TypeCheckVis
 
     public override void PostVisit(ConstDeclaration constDeclaration)
     {
-        base.PreVisit(constDeclaration);
+        base.PostVisit(constDeclaration);
 
         (IdentifierSemanticToken token, ConstValue constValue) = constDeclaration.ConstValue;
 
diff --git a/Canon.Core/SemanticParser/PascalType.cs b/Canon.Core/SemanticParser/PascalType.cs
index 19673d7..dc7e3e5 100644
--- a/Canon.Core/SemanticParser/PascalType.cs
+++ b/Canon.Core/SemanticParser/PascalType.cs
@@ -31,7 +31,7 @@ public abstract class PascalType : IEquatable<PascalType>
             return false;
         }
 
-        return IsReference == other.IsReference;
+        return true;
     }
 
     public T Convert<T>() where T : PascalType
diff --git a/Canon.Core/SemanticParser/SymbolTable.cs b/Canon.Core/SemanticParser/SymbolTable.cs
index cb38e95..eb80130 100644
--- a/Canon.Core/SemanticParser/SymbolTable.cs
+++ b/Canon.Core/SemanticParser/SymbolTable.cs
@@ -44,7 +44,16 @@ public class SymbolTable
     /// </summary>
     /// <param name="symbol">欲添加的符号</param>
     /// <returns>是否添加成功</returns>
-    public bool TryAddSymbol(Symbol symbol) => _symbols.TryAdd(symbol.SymbolName, symbol);
+    public bool TryAddSymbol(Symbol symbol)
+    {
+        if (_symbols.ContainsKey(symbol.SymbolName))
+        {
+            return false;
+        }
+
+        _symbols.Add(symbol.SymbolName, symbol);
+        return true;
+    }
 
     /// <summary>
     /// 尝试从符号表极其父符号表查找符号
diff --git a/Canon.Core/SemanticParser/TypeCheckVisitor.cs b/Canon.Core/SemanticParser/TypeCheckVisitor.cs
index 4770f41..bd21320 100644
--- a/Canon.Core/SemanticParser/TypeCheckVisitor.cs
+++ b/Canon.Core/SemanticParser/TypeCheckVisitor.cs
@@ -22,7 +22,7 @@ public class TypeCheckVisitor(ICompilerLogger? logger = null) : SyntaxNodeVisito
 
     public override void PreVisit(ConstDeclaration constDeclaration)
     {
-        base.PostVisit(constDeclaration);
+        base.PreVisit(constDeclaration);
 
         (IdentifierSemanticToken token, ConstValue constValue) = constDeclaration.ConstValue;
 
diff --git a/Canon.Tests/SemanticTests/SymbolTableTests.cs b/Canon.Tests/SemanticTests/SymbolTableTests.cs
index af8ab8b..e89038d 100644
--- a/Canon.Tests/SemanticTests/SymbolTableTests.cs
+++ b/Canon.Tests/SemanticTests/SymbolTableTests.cs
@@ -54,4 +54,14 @@ public class SymbolTableTests
         Assert.True(table.TryGetSymbol("temperature", out Symbol? temp));
         Assert.Equal(PascalBasicType.Real, temp.SymbolType);
     }
+
+    [Fact]
+    public void DuplicatedTest()
+    {
+        SymbolTable table = new();
+
+        Assert.True(table.TryAddSymbol(
+            new Symbol{SymbolName = "a", SymbolType = PascalBasicType.Integer, Const = true}));
+        Assert.False(table.TryAddSymbol(new Symbol{SymbolName = "a", SymbolType = PascalBasicType.Boolean}));
+    }
 }