Error message
In the message, %s marks a value that the engine fills in when it raises the error.
The story
You gave the same parameter twice to the same being, and the engine does not know which one you meant. Picture writing a word and asking for it to be red, and then blue, in one breath. Which is it? A parameter may appear once per being. The first example in the rule shows the problem: write the letter a with a red foreground, and again with a blue foreground. Two colours, one being. Refused. But here is the good part. If you wrote a, with red, and then b, with blue, those are two different beings, and each one has its own colour. That is perfectly fine. The whole thing is checked at parse time, before anything runs, so you hear about it early. The message names the repeated parameter and the context. To fix it, decide which value you actually wanted, and remove the other. Or, if you really have two beings, make sure each one has its own group, with its own value written once. One being, one value per parameter. Clean and clear.
Text from the Rush engine's error catalogue.