title: ASP.NET Identity: attempting to add user with existing ID
body: <p>ASP.NET Identity version 1.0</p>
  
  <p>What is the default behavior of UserManager.CreateAsync(user, password) method call if we are trying to add an user with ID already existing in database?</p>
  
  <p>I'm asking because some time ago the following line of code</p>
  
  <pre><code>IdentityResult result = await UserManager.CreateAsync(user, password);
  </code></pre>
  
  <p>returned an IdentityResult object with not empty Errors list saying something like "User with ID [ID] already exists".</p>
  
  <p>But now (we didn't do much changes since that) we've got an exception when attempting to insert new user record in the database. 
  In both cases UserManager.CreateAsync calls FindByNameAsync in our IUserStore implementation and that method finds and returns a correct User object.</p>
  
  <p>How can I see the internal implementation of UserManager.CreateAsync method? 
  Does it check new User object for existence before calling CreateAsync of IUserStore?
  I guess it does (since it calls FindByNameAsync but it seems it does not consider found User object as "existing".</p>