The specified Active Directory user already exists as a CRM user

Problem Description:

You won't be able to add the Active Directory user to the CRM users list from Settings->Administration

It throws the below error;

Unhandled Exception: System.ServiceModel.FaultException`1[[Microsoft.Xrm.Sdk.OrganizationServiceFault, Microsoft.Xrm.Sdk, Version=5.0.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35]]: The specified Active Directory user already exists as a CRM user.Detail: 
<OrganizationServiceFault xmlns:i="http://www.w3.org/2001/XMLSchema-instance" xmlns="http://schemas.microsoft.com/xrm/2011/Contracts">
  <ErrorCode>-2147214036</ErrorCode>
  <ErrorDetails xmlns:d2p1="http://schemas.datacontract.org/2004/07/System.Collections.Generic" />
  <Message>The specified Active Directory user already exists as a CRM user.</Message>
  <Timestamp>2015-05-21T15:20:36.6981759Z</Timestamp>
  <InnerFault i:nil="true" />
  <TraceText i:nil="true" />
</OrganizationServiceFault>

Cause: May be SysAdmn has deleted the created user by mistake or something else might have been happened.

Solution:

As per below article, get the user id and crm user id. And perform the Microsoft unsupported tasks like delete the entries in 

SystemUserOrganizations where CrmUserId = 'XXXXX'

SystemUserAuthentication where AuthInfo like '%.......%'

And close all the connections to the SQL server and clear the cache in IE for CRM and wait for few minutes and then try to Add the AD user to CRM and it should work.

Second part of the problem would be if the AD user was created and deleted and recreated, update the CRM systemuser with that particular ActiveDirectory GUID for the systemuser in the spotlight, should resolve the problem.

http://blogs.msdn.com/b/ukdynsupport/archive/2009/07/16/authentication-when-you-are-adding-a-new-user-to-crm-or-a-user-to-a-new-organization-in-crm.aspx