I don't know why my first post looks like that. Here it is again:
For some reason a few days ago, Address Book on my work MacMini sprouted an extra list, so that it now has an 'iCloud' list and an 'On My Mac' list. I don't know why it did this: it hasn't happened on my home computer.
I'm guessing that when the extra list (whichever that might be) was originally just a duplicate of the first. If it had stayed that way, then there would be no problem. However, in the past two days I had no classes, so I spent a lot of time sorting out my contacts: deleting some and adding a lot. It seems that some of these changes have been made to the 'On my Mac' list, and some to the 'iCloud' list. And only the 'iCloud' list has been synced to iCloud.com and my other computers.
I didn't notice this till yesterday morning. This sudden sprouting of an extra list has happened to a few people, and the general advice is just to delete all the addresses in it, and it will go away. But if I do that, then I'll lose all the changes and additions that I made. So I dunno what to do about it. I suppose I'm just going to have to pick through it and transfer stuff to the iCloud list bit by bit (and yet again go through the list to weed out the dead people).