Supress Prompts when file exists in a Collection
Copy WHENEXISTS=skip
It works when the folder is standard windows folder, any way to do it for collections?
Any tips on how to avoid error saying "the item is already in the collection"?
In my opinion, this information is more often useless than useful.
Why anyone copies an item to a collection? Because they want that item to be in a collection (seems quite obvious, right?). So if the item is already there, why bother and inform user that it's already in the collection?
It reminds me of error that can be seen on some websites: "Unable to log out. if you want to log out, log in first". 
If the goal is already achieved, don't disturb user with useless dialogs.
A simple option for that in the preferences would be sufficient, i would say, so that the users can decide. There is no harm in not being notified, if an item is already present in a collection, i agree.
I think it's useful in case it alerts you to the fact you're doing something wrong/unexpected (e.g. dragging files from the wrong folder that you'd already put in the collection, and not new ones that you haven't).
You can also click Skip All once to deal with it, so it doesn't seem a big deal to me either way.
What are people doing that they run into this so often for it to matter?
I'm trying to use collections to categorize files. If I try to copy the same file to same collection twice and there's no information the file is already there, nothing wrong happens - I wanted to copy it there anyway.
When I see a dialog window with red icon, which usually indicates error, I'm starting to think of what could have gone wrong instead of stick to what I'm doing now.
Eg. I have collection with music or movies to watch later. I may want to copy a file to that collection not realizing it's already there (because I was planning to "watch it later" at some point in the past, ie. much earlier).
Hmm, I guess that makes sense.
(Although, if it's anything like my "to watch", "to read", etc. lists, and I'm guessing it is if you add things over time but don't look at the list often enough to remember what's on it, then it might as well be a black hole. There's never time to look at those lists and always too many things we know about to watch/read/do before we need to check them. 
)