JSON duplicated properties validation via Jackson
JSON specification indicates duplicates are not consider valid, but parsers are not required to do anything about them. From practical perspective, keeping track of all seen properties adds overhead, which may not make sense at streaming parser level.
As to Jackson, it used to have duplicate detection at data binding level, but I think that is not enabled at this point. It could be added fairly easily when dealing with Maps.If this is something you would want, filing a feature request or asking on user list might make sense (esp. to see if others would want this feature too, making it more likely to get added soon).
If all you want to do is just validation, you could create a Map subclass, make it throw exception on duplicate. Or, just set a flag in sub-class that you can check if you prefer.
JSON does not support duplicated properties. So if your input is guaranteed to be valid JSON you don't have to check for them.