I have seen this a lot and often wonder. A few friends have had their second bubs in the same month as the first.. I really really wanted to avoid it happening as DS and DH already have their birthdays really close! Next one is due a good month before though, thankfully!
A close friend recently had twins after her first bub and the twins were born 2 days before her eldests birthday!
My older sibling and I have birthdays 2 weeks apart. And my brother in laws is in between that.