Error message
In the message, %s marks a value that the engine fills in when it raises the error.
The story
You asked data add to put in a new item, but that key is already taken. Remember the two doors. Add is for new items. Update is for changing existing ones. They are mirror images on purpose, so that overwriting a value has exactly one door, and nobody replaces something by accident. If add found the key already there, or even if your own call named the same key twice, it refuses instead of silently replacing a value you might not have known about. Picture a hotel desk: if room twelve is occupied, you do not push the guest out, you either pick another room, or you ask explicitly to change the guest. The message tells you the collection name and the key that collided. Your fix depends on your intention. If you meant to change the value, use data update with the same key. If you meant to add something new, pick a new key. And if the call named the same key twice, remove the duplicate. Add never overwrites, and that promise protects your data.
Text from the Rush engine's error catalogue.