Add opt-in failure for duplicate elements when deserializing JSON arrays into Set types

[dpkass]

Is your feature request related to a problem? Please describe.

When Jackson deserializes a JSON array into a Set, elements considered duplicates by the target Set are silently discarded.

This is normal Set behavior, but it makes deserialization lossy and can hide invalid input. Applications with Set-valued request properties may want to reject duplicate array elements instead of silently canonicalizing them.

Describe the solution you would like

Please consider adding an opt-in deserialization setting that fails when the target Set rejects an array element as a duplicate.

Existing behavior should remain the default for backward compatibility.

The setting could potentially be exposed globally through a DeserializationFeature, per property, or both. The resulting input-mapping error should include the duplicate element array index where possible.

Usage example

The appropriate configuration API should be agreed with the maintainers first.

Additional context

I observed this with Jackson Databind 2.19.2 and could not find an existing issue or pull request for opt-in duplicate rejection during Set deserialization.

I would be willing to contribute an implementation once the desired API and scope are agreed.

[pjfanning]

There are a few things that you could try instead of getting a Jackson enhancement that could take months to release and may only be added to the latest 3.x release.

One option is this JsonDeserialize tag trick (not tested by me but should work in theory):

public class StrictSet<E> extends LinkedHashSet<E> {
    @Override
    public boolean add(E e) {
        if (!super.add(e)) {
            throw new IllegalArgumentException("Duplicate element: " + e);
        }
        return true;
    }
}

public class Config {
    @JsonDeserialize(as = StrictSet.class)
    private Set<String> tags;
}