Visual Studio 2012: Unable to attach the process. A debugger is already attached

You have 2 instance of VS and two entries in IIS running on the same application pool, for instance Both Server and Intranet are running on the same application pool.

=> Solution open IIS :

° Select the site and click 'Basic Settings' : Here you can check/select the application pool used.

° Click on the "Application Pools" node (right under your computer name) => You get a list of defined application pools.

° Right click under the last one defined to open the context menu and select 'Add Application Pool...' Create a new name and give it the same parameters that the one that was used by your site.

° Select the 'Basic Settings' of one of your sites and change its application pool to the new one just created. => Now Server and intranet have different application pools and your can debug the both at the same time.

Hope this helps.

For me, even after disabling the Debug Diagnostic Tool v2.0 this issue didn't fix. So, I rebuild my webservice, pushed the DLLs on the IIS and restarted the it, which fixed the issue for me. This issue may occur when you make changes in the your web service which is hosted on your local IIS and those changes are not pushed.

I have installed Debug Diagnostic Tool v2.0 and as a result I have Debug Diagnostic Service which is started automatically and attached to one of w3wp processes. After turning off and disabling this service all works fine. So the general rule: if you get such error check processes in task manager which can capture your w3wp process.