One thing you discover after having twins is that no two toys are exactly the same. They may be factory made, they may look the same to you, but little girls notice things.
Two soft toy kittens I once bought seemed identical. However, a single stitch different on their mouths meant one became known as Sad Cat, the other as Happy Cat
For their third birthday one of the things they asked for was Barbie mermaid dolls. You put batteries in them and they really swam. Sounded cool and looked great in the ads.
The girls were delighted when they saw them. I half filled the bath and began the long task of unwrapping. I finally inserted batteries in the first and put her into the water. She began to gracefully do lengths of the bath and I set to work getting the next one ready.
I placed the second one into the bath and we waited for her to begin. Instead she began to shudder and shake, rooted to the spot. She appeared to be having a seizure, while the other one swam happily back and forth.
That was when I decided to buy three of everything.