I have just taken a new Alloy site and first upgraded it to the latest version of EPiServer and then added EPiServer Find to it.
Doing that I got this error:
Cannot add duplicate collection entry of type 'add' with unique key attribute 'name' set to 'ExtensionlessUrlHandler-Integrated-4.0'
When I open up web.config I can see that I have two items for that handler that looks like this:
I think the one with verb="*" is the lastest one.
I solved it by removing one (the one with all verbs declared) but could be good to know for you and anyone else that get this error
Probably it is related to Microsoft.AspNet.WebApi.Core NuGet package upgrade from version 4 to version 5 when EPiServer Find packages are being installed.
For some reason newer handler definition (verb="*") is added instead of replacing the older handler definition (verb="GET,HEAD,POST,DEBUG,PUT,DELETE,PATCH,OPTIONS").