I'm torn on this.
On the one hand, it's incredibly rude to say that people who have second baby showers shouldn't just because you wouldn't. Or that they're being grabby, greedy, etc. Unless you know them personally and know what their reasons are for having multiple showers and you know that it's because they just want a bunch of new stuff and it has nothing to do with baby #2, then don't judge and assume the best! You wouldn't want people assuming the worst of you so don't assume the worst in them. And maybe it wasn't their idea to throw a second shower? The ladies group at my church hosts small baby showers for every baby, regardless of how many kids the parent already has. Some people just like to celebrate another life on the way and it's more of a social hour than a gift giving hour. And who knows, maybe there is a reason beyond greed for the people that do have a large registry list and are 'asking' for gifts. Maybe something happened and they don't have all the baby stuff they use to have. Maybe baby #2 was a surprise and they had already gotten rid of all/most of their baby things from baby #1 or it's just taken that long to get to baby #2. My parents had to have a whole new baby shower for me because my closest sibling was 6 years older than me and they had gotten rid of most of their baby items by giving them to friends having babies.
That said, people shouldn't rely on baby sites to tell them if they can or cannot have a second shower. If you want one, have one. If you feel torn, ask other moms or even your mom if they think it would be acceptable or what makes a second shower acceptable and what doesn't. But again, some people might be looking online for answers because someone offered to throw a second shower and they don't know if (for face value) they should turn it down or be considerate and thankful and accept the second shower. I hate when people guilt someone into doing or not doing something that they had wanted to participate in or say. It's my number one pet peeve whether it was intentional or not.
Also, if you're having a different gender baby for baby #2 and want to stick with a gender specific theme as opposed to making it as gender neutral as possible, I see nothing wrong with having a real baby shower for that baby for gifts that are gender specific or things that will be needed to deal with two young children at the same time that you don't already have.