Activiti/Activiti · error · JSONException

Duplicate key \"" + key + "\"

Error message

Duplicate key \"" + key + "\"

What it means

putOnce(key, value) adds a pair only if the key is not already present; when opt(key) returns an existing value it throws JSONException("Duplicate key \"<key>\""). It is used by constructors building a JSONObject from another source that should have unique keys.

Solutions

1. Check opt(key) == null before calling putOnce, or use plain put(key, value) to intentionally overwrite
2. Deduplicate the source map before constructing the JSONObject
3. Decide the merge policy: skip, overwrite, or fail — and use the matching API (putOnce, put, or explicit check)
4. Fix upstream data so keys are genuinely unique

Example fix

// before
json.putOnce("id", idValue); // throws if "id" already set
// after
if (json.opt("id") == null) {
    json.putOnce("id", idValue);
} else {
    json.put("id", idValue); // explicit overwrite policy
}

if (json.opt(key) != null) { throw new IllegalStateException("duplicate key: " + key); }

boolean isKeyFree(JSONObject src, String key) { return src.opt(key) == null; }

try { json.putOnce(key, value); } catch (JSONException e) { log.warn("duplicate key " + key + " — skipping", e); }

When it happens

Trigger: Calling putOnce with a key that already exists in the JSONObject, or constructing a JSONObject from a Map/bean where two entries collapse to the same key name (e.g. bean property name collision).

Common situations: Merging two JSON documents with overlapping keys; building from a map where case differences were expected to be distinct but were normalized; reflecting over beans producing duplicate property names.

Understand the failure class

Background: Schema validation failed / invalid input schema: payload rejected because its shape doesn't match the expected schema — this error's family across 28 libraries.

Related errors

• Bad value from toJSONString: " + o
• e
• Error deserializing json info for process definition
• JSON does not allow non-finite numbers.
• JSONObject[" + quote(key) + "] is not a JSONObject.

AI-assisted analysis of Activiti/Activiti@56435b1a97 (2026-09-09). Data as JSON: /api/errors/05422174f28bef68. Report an issue: GitHub.

Appendix: source

Thrown at activiti-core/activiti-engine/src/main/java/org/activiti/engine/impl/util/json/JSONObject.java:1004

        } else {
            remove(key);
        }
        return this;
    }

    /**
     * Put a key/value pair in the JSONObject, but only if the key and the value are both non-null, and only if there is not already a member with that name.
     *
     * @param key
     * @param value
     * @return his.
     * @throws JSONException
     *           if the key is a duplicate
     */
    public JSONObject putOnce(String key, Object value) throws JSONException {
        if (key != null && value != null) {
            if (opt(key) != null) {
                throw new JSONException("Duplicate key \"" + key + "\"");
            }
            put(key, value);
        }
        return this;
    }

    /**
     * Put a key/value pair in the JSONObject, but only if the key and the value are both non-null.
     *
     * @param key
     *          A key string.
     * @param value
     *          An object which is the value. It should be of one of these types: Boolean, Double, Integer, JSONArray, JSONObject, Long, String, or the JSONObject.NULL object.
     * @return this.
     * @throws JSONException
     *           If the value is a non-finite number.
     */
    public JSONObject putOpt(String key, Object value) throws JSONException {

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