I am using Twitterizer which works great. I have a website (.NET 4, C#) where a user visits and signs-in... I store tokens e.t.c everything works great.
The question is how can I avoid asking the user to grant access to my app via twitter since I already did this once the first time (and let's say user is already signed in twitter in a different tab on my browser)?
Use http://api.twitter.com/oauth/authenticate?oauth_token=zyx instead of /oauth/authorize?oauth_token=xyz.
Read more on the Sign in with Twitter and GET oauth/authenticate documentation.