Mail: Warn when adding an existing label
I am not sure if this is very important but I just tried to create a new label in order to better categorize my e-mails and I accidentally picked a color that is similar to another label. So, without thinking of it very much, I created a new label with the same name and assigned a different color.
The old label (that had the exact same name but a different color) was left intact (I wasn't warned that another label with the same name exists) and also when picking one label, the other one is also picked (which seems like a bug).
I know that I can delete the wrong label from the preferences menu, but I just wanted to point it out.