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").
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