A couple of months ago I was making some calls to the SharePoint Lists Web Service, and I found that I was getting the same item back multiple times. This puzzled me – I mean, it was clearly by design, but why would anyone want a copy of the item for each value selected in a multiple-selection lookup?
It hit me last weekend (like a bolt from the blue) – you could use this to support Group By clauses on queries on a list. Yes, I know, SharePoint itself doesn’t support this functionality, but you could use the behaviour that I’d seen to support this. It’s the only explanation I’ve found that makes sense.
Now, can you turn off this behaviour, so you only get one item. That would be a lot more useful for what I was trying to do, although I now get why you might want that behaviour. Well, I don’t see a query option for it in the documentation, and cracking the service open in Lutz Reflector doesn’t show an option for it either.
I guess we’ll just have to accept the GetListItems command in the Web service API supporting Group By functionality, while SharePoint’s user interface itself does not. It’s a bit weird though that there isn’t an option I can find to turn it off though, and the Object Model behaves quite differently – it only returns one ListItem, and is unaffected by the multi-select lookup.