I built solution, created application in IIS and mapped it to the application folder. It works fine. Then I go to "Attach to Process", there are two w3wp.exe processes in list, but for one of them I get error "Unable to attach the process. A debugger is already attach." I've googled it but I can't find solution for my problem.
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.