You could have 1 non-standard map to multiple standard concepts, that is fair game. This is usually found in cases where one code is associated to multiple conditions, so the concept_relationship will map the single code to each specific condition that is packaged in it.
I think the rule you are thinking is that there's exactly one standard concept for a 'medical idea', meaning you won't find multiple concepts for Still's disease.
In the case you brought up, it looks like the ICD10 code M06.14 is mapping over to 2 concepts, one for Still's Disease, the other for RA of hand joint. From what I read, Still's shares many characteristics with RA so perhaps that is why there is the dual mapping. But, I checked the vocab and it doesn't look like Still's rolls up to anything under RA, so the mapping isn't redundant.
I can't give you the actual reason why this particular concept got mapped to two conditions. Most likely the vocabulary team got that mapping from some source, or you did find a mapping error. But, to your original question, it is valid to have a non-standard concept map to multiple standard concepts.