It is possible to assign the same hotkey for "same context" actions. Just like this:
![Image](https://forums.factorio.com/images/ext/e4630552e55b1202c7d8816b780439d6.png)
As far as I could get from tests, de facto SHIFT+1 got assigned only for Quickbar 6.
When I had different items in slots, only 6th slot returned an item for SHIFT+1.
Technically, this is not a bug, as it is possible to have the same button for multiple actions, yet, it would be nice to perform check for hotkeys collisions and prompt user if such exist.