Ereo
Helper
Doesn't really help the issue, but museum and collection tab are separate from each other. Collection tab is supposed to be grey until you find the item, whether you give it to the museum or not has nothing to do with it.When I find a 2nd instance of the item, it is showing me the tooltip describing it rather than the standard “Gunter can tell you more...” that you would expect if it was still outstanding. I also tried taking my second Elven Jewelry into the museum to see if that would resolve the issue but was told I had nothing to donate.
So it is possible to still get the "Gunther can tell you more about it" description for an item already in your collection tab.
The other way round shouldn't be possible though, anything you donated to the museum should be in your collection, because you had it.