In a tournament, you'd get a TD ruling and some strange looks, and I think it depends on the TD and their house/tournament rules. If I were TD, I think I'd rule that as invalid and give you no build. However, if you order Build A Bre and Build A Bre, you'd get (one) A Bre. When I've run a tournament, the rule I've used (to be as nice as possible to newer players) is that any order which admits only one legal interpretation is considered valid, and orders that admit multiple legal interpretations are invalid. So, for example, I've had a player who liked to write "Norw." as the destination of a unit. If there was only one territory beginning with "Norw." to which that unit could move legally, it was a valid order and an admonition to be careful (F Edi - Norw.). However, if there were multiple (F North Sea - Norw.), then it was invalid.
However, webDip is a bit different, because webDip is intentionally set up to avoid allowing the possibility of invalid or ambiguous orders (which is unfortunate those times when an invalid order would be convenient...not that such times exist or anything!). So, I think on webDip, as a new player I'd tend to assume that anything which makes it past the parser is valid, and thus giving either an army of a fleet instead of neither seems like a more reasonable decision to me. The better answer, of course, is to have the website reject such orders. I believe that when I choose a unit type, the site repopulates the options for where to put it, so that I can't ask for a fleet in Moscow. Doing something similar when I select a build destination would be a better solution, IMO.