count the number of user that has the same username and add the user if the count is 0.
for example
var count = dataU.UsersTables.Count(u=>u.UserName == usernameyouwanttocheck);
if(count==0)
{
//add user
}
else
{
//alert user saying user exists
}
if I were you I would make repository and create a function that checks if the user exists or not and call that function from controller.