Added a check if the current value already exists (Closes #5)

Assignment 1/Program.cs

@@ -233,6 +233,11 @@ namespace Assignment_1
                     {
                         throw new ParserException("Cannot assign a value to a reserved constant", 0, keyEndPos - (key.Length + 1));
                     }
+                    else if (Symbols.ContainsKey(key) && !checkExist)
+                    {
+                        // key already exists, remove it 
+                        Symbols.Remove(key);
+                    }
                     source.Position = keyEndPos;
                 }
                 return key;