ASP.NET 4.0 - Remove index.aspx from url / Duplicate content?

Question

I have a big problem on my website (ASP.NET 4.0 with SSL Certificate) 3 months ago.

The thing is when the user try to access using www - the website refirect to https://xxxx.es

However when the user try to access to the website without www - it redirect to https://xxxx.es/index.aspx

Google says that it is duplicatr content.

I need to remove the index.aspx in the second case or redirect to the index.aspx in the first case. I prefer remove index.aspx ...

I have tried to resolve the problem in many different ways:
- Using Plesk
- Using web.config - creating rules
- Using Redirect
- Using Google Wemaster Tools - They say that the configuration is ok

But any of them solve my problem.

I hope you can help me.

Thanks. :)