Where does error CS0433 "Type 'X' already exists in both A.dll and B.dll " come from?

Removing the class files from the App_Code folder, and placing them directly under the website, solved this issue for me.

Reference : https://support.microsoft.com/en-in/help/2028526/building-an-asp-net-project-in-visual-studio-results-in-compiler-error

When building an ASP.NET project using Visual Studio, you may randomly see an error message similar to the following:

Compiler Error Message: CS0433: The type 'ASP.summary_common_controls_notes_ascx' exists in both 'c:\Windows\Microsoft.NET\Framework64\v2.0.50727\Temporary ASP.NET Files\Book_Details\abc12345\def8910\App_Web_msftx123.dll' and 'c:\Windows\Microsoft.NET\Framework64\v2.0.50727\Temporary ASP.NET Files\Book_Details\abc12345\def8910\App_Web_msfty456.dll'

Description: An error occurred during the compilation of a resource required to service this request. Please review the following specific error details and modify your source code appropriately.

Source Error: Line 100: Line 101:
New Notes Line 102:
Line 103:
1450 Line 104:

Summary.

Source File: d:\http\post\publisher\default.aspx Line: 102

Common scenarios where this error can occur are discussed below

Scenario 1

Description: A common cause is when there are two assemblies in the same web application bin folder containing two class definitions but that have the same class name. This can happen if more than one Default.aspx was compiled into a single assembly. Commonly, this occurs when the Master page (Default.master) and the Default ASPX page (Default.aspx) both declare a _Default class. Solution: Change the class name of the master page (from _Default in most cases) and rebuild the project. It is important to resolve any naming conflict between classes.

Scenario 2

Description: The Reference Paths in Visual Studio is used to specify the folder path for assembly references used by the project. It is possible that the path contains an assembly that contains the same class name. It may be that there are multiple References added to the same assembly (possibly different version or name) causing a naming conflict.
Solution: Remove the old version reference. To do so, in Visual Studio right-click your web site and check the "References" in the properties.

Scenario 3

Description: By default, when an ASP.NET Web application is compiled the compiled code is placed in the Temporary ASP.NET Files folder. By default the access permissions are given to the ASP.NET local user account, which has the high-trust permissions needed to access compiled code. It is possible that there were some changes in the default permissions causing versioning conflicts. Another possibility would be that anti-virus software could be locking an assembly inadvertently. Solution: Clear the Temporary ASP.NET Files Folder of all contents.

Scenario 4

Description: When the batch attribute in the web.config is set to True it eliminates the delay caused by the compilation required when you access a file for the first time. ASP.NET precompiles all the un-compiled files in a batch mode, which causes delays the first time the files are compiled. Turning off batch compilation may expose masked compilation errors that may exist in the application but are not reported. However more importantly for this issue it tells ASP.NET to dynamically compile individual .aspx/.ascx files into separate assemblies instead of into a single assembly. Solution: Set batch=false in the section in web.config. This should be considered a temporary solution as setting batch=false in the compilation section has a significant performance impact on build times for the application in Visual Studio.

Scenario 5

Description: Modifying the web.config file for an ASP.NET application or changing a file in the bin folder (such as adding, deleting, or renaming) causes the AppDomain to restart. When this occurs all session state is lost and cached items are removed from the cache as the website restarts. It is possible that the problem is caused by an inconsistent state in the web application. Solution: Trigger an AppDomain restart by touching (editing) the web.config file.

Scenario 6

Description: You can store source code in the App_Code folder, and it will be automatically compiled at run time. The resulting assembly is accessible to any other code in the Web application. The App_Code folder therefore works much like the Bin folder, except that you can store source code in it instead of compiled code. The class will be recompiled when there is a change in the source file. If there is conflict due to an outdated assembly then forcing a recompile may resolve the problem. Solution: Touch a file in the Bin or App_Code folders to trigger a full recompilation.

There are quite a multiple of reasons for this. And most of the ones mentioned above apply to different scenarios. What I noted is that the error occurs ONLY when authentication is set to something else other than 'None'. For my testing purposes I will set this off and it works.

I got re-directed here when clicking on the first Google hit when clicking on the error URL for CS0433, specifically,

The type 'Package' exists in both 'Windows... Version=N.N.N.N, Culture=neutral, PublicKeyToken=null, ContentType=Windows...' and 'Windows..., Version=255.255.255.255, Culture=neutral, PublicKeyToken=null, ContentType=Windows...'

Instead of outlining all the things I did to fix it, let me tell you what I did that broke it. I went to update the NuGet packages for a repo that needed a code update. The packages were pretty old (1 year or so) and all I initially tried to do was update it for a C# project.

Sometime between starting that process and encountering this error, I somehow downgraded the version of the C++ projects in that SLN to target 15063. I also noticed that the C# project had both the TargetPlatformMinVersion and TargetPlatformVersion newly set to 10.0.17134.0

The only thing I had to do to "fix" it was change the TargetPlatformMinVersion to a higher version than the TargetPlatformMinVersion for the C# project. Modifying the C++ project to either version didn't change behavior. I'm not sure why this suddenly stopped working but hopefully someone similarly blocked might get out of a pickle using similar strategies.

In addition to trying 2Toad's answer, I also ended up having to close Visual Studio and delete my .vs folder. After that, everything built correctly.

Incidentally, the error I was encountering didn't specify my Temp folder at all, but was referencing something else that was obviously system-generated. I neglected to save the specific error :\

if no other solution worked, then just rename the inherits class of that problem causing aspx file and aspx.cs file to a new name, then rebuild the solution. then the issue will be solved for sure. this only worked for me.

eg :

in the aspx file do the following change the inherits class name to Defaultnew

<%@ Page Title="" Language="C#"  MasterPageFile="~/Main.master" AutoEventWireup="true" CodeFile="Default.aspx.cs" Inherits="Defaultnew" %>

in the aspx.cs file rename the class to the same as used in aspx file

using System;
using System.Collections.Generic;
using System.Web;

public partial class Defaultnew : System.Web.UI.Page
{

Close Visual Studio and clear bin and obj folders in your project directory