WPF ListView slection changed doesn't removing an old item

I have a strange behavior with my WPF ListView Control.

ListViews ItemSource is Observable collection.the ItemSource is updated periodically.

When I'm selecting one of the item and then selecting other item and no item updated, everything is OK.

But when I'm selecting an item witch is updated while I'm standing on, then selecting other item, now I have two items selected instead of one.

When I'm looking with the debugger, I see the event args of SelectionChanged event. I see that added item is OK but no removed item.

Anyone knows what's the problem?

Thanks!

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