I would love it if when multiclassing and a spell is available on both spell lists, if there was an indication that that spell has been chosen on another list. 

As it is now, if you select a spell on List A as "Known" or "Prepared" and look at List B with the same spell, there is no indication and you can select the spell a second time if not paying close enough attention. Even if you are paying close enough attention, it requires a lot of needless scrolling or opening another window to compare spell lists. A tag of *Spell Known/Prepared next to the applicable spells on all lists would be really handy. Or perhaps just greying out the "Learn" button on all lists once the spell is selected on another list.

I wouldn't go so far as to grey it out, but an indicator would be nice. Maybe put an asterisk or triangle next to the learn button.