User not found Exception when using web.ensureuser function

Hi,

This is happening when we are doing performance testing by adding more number of users concurrently after resolving ensureuser function. For which user we are getting the user not found issue we can able to add the same user directly in the site.

Note: using saml provider we are adding the user for example - i:0ǵ.t|domain|username

And for normal run of code its working fine it can resolve the user by using ensureuser function. only the problem when we are resolving the user by running concurrently for more users. 

please do the needful.Thanks