I think it would be very useful if when you select an item through the keyboard, that has multiple on that slot, like multiple grenades or tnt, the first item would always be selected.
Example:
Currently: You have two types of grenades on you. Pressing 5 once will select the first type of grenades. If you then switch to another weapon and switch to grenades later the second type will be selected instead of the first.
With this suggestion: Even if you have selected grenades previously, as long as you switched to some other equipment in the mean time you will always select the first type of greandes as long as you press 5 once.
This would help, so that you don’t have to remember that you selected grenades at some point, but decided not to throw them.