This is exactly where I was when I found out I was pregnant with second son, my first son was only 11 months old, so I fell pregnant when he was 10 months. We'd just moved house and was coming out the other side after rather bad PND. Unplanned and a complete shock. They're 20 months apart.
I cried and cried for about a week, and really thought I couldn't go through with the pregnancy. But with lots of talks with husband etc, we decided that we did want two children anyway, and although the age gap is not what we wanted, now the boys are 2 and 4, best of friends and I couldn't imagine life any different.
However at the time of finding out I was having another baby I was terrified - I'd had an awful labour, homebirth gone wrong, breastfeeding didn't work out, which led to PND which wasn't treated properly... I knew I was going through with a pregnancy I didn't plan or expect, and was horrified at having to go through it all again, so I did feel pretty low throughout and worried myself sick about the impact on my first son, who I wanted to spend more one to one time with than we actually got. Turns out eldest adored his little brother from day one and learned to share very early on. The benefits of them having eachother far outweigh any of my intial concerns.
All turns out that even though I had en EMCS, another breastfeeding nightmare, and a second baby who was lactose intolerant and quite a difficult child, all of that is now in the past - the boys have a constant playmate and are inseparable. I'm so proud of them. Personally, plus sides are we have had our children close together and am not having any more. The baby bit (which we found hard) is done. For them, hopefully they will remain close through life.
It has been hectic, our relationship has taken a back seat, and it's all gone by in a blur really! Eldest starts school in Sept, I do deep down wish we'd had more quality time together, but it is a crazy but happy household mostly!
Good luck, whatever you decide will be right for you and your family. An 18 month age gap is more common than you may think! I was the first out of all my ante-natal friends to fall pregnant and felt like everyone would think I was mad, but you'll meet more and more people with that age gap, trust me!