Gives an error of Multiple assemblies with equivalent identity have been imported for "System.Web.dll"

I added "System.Web.dll" in my project but it gives me an following error:

 Microsoft (R) Visual C# Compiler version 2.9.1.65535 (9d34608e)
 Copyright (C) Microsoft Corporation. All rights reserved.
 error CS1703: Multiple assemblies with equivalent identity have been imported: 'E:\TestProject\Assets\DLLS\System.Web.dll' and 'C:\Program Files\Unity\Hub\Editor\2019.4.35f1\Editor\Data\NetStandard\compat\2.0.0\s$$anonymous$$ms\netfx\System.Web.dll'. Remove one of the duplicate references.

I removed dll from assets folder but no used. It gives me another error. It works perfectly in Unity 5.6.7, but not working in Unity 2019.4.35f1

Api Compatibility level : .Net Standard 2.0