I don't see it as an issue, saves them fighting over toys. Although my 2 will still fight over 2 toys that are exactly the same. How old are your 2 ? I have found as they get older they really like to have their own possessions, so it doesn't matter so much if they have 2 of the same toy, they are just happy to have something that is all theirs
Never got same books but got lots of the same toys tbh never bothered me as I'm sure having 2 of everything stopped many a squabble! & as the kids stopped playing with it as much i'd give one to charity shop or to friends that had use for it.
If it had been an issue I probably would have suggested they only buy one of something as the 2nd one is often not used ... imo people tend to rethink quickly their choice in purchases if they think they are wasting money!