It's probably wise to buy something that won't matter if it's doubled up on. Don't want too many photo albums etc....and you can get some quite ordinary and useful things that are prettily gift wrapped or boxed to make them look really....gifty......☺
i dont like baby showers that are planned, its almost like 'hey buy me a present'. my sis held a surprise one for me which was such a shock i almost had the baby lol. i didnt get presents as they gave me gifts once he was born but we had games and lots of cake :-)
I'm with Boxie in that these are a racket aimed at yet more consumerism. But if you would like to get the prospective mum something always useful, a packet of cotton muslins, baby wipes, and first size nappies is always useful.
Or you could pledge to come and do night time / twilight duty while the dear new parents realise what's happened.