StatelessForm 内での使用は上手く動作しませんでした。

SyncTokenField.java

import lombok.extern.log4j.Log4j2;
import org.apache.wicket.devutils.stateless.StatelessComponent;
import org.apache.wicket.markup.ComponentTag;
import org.apache.wicket.markup.html.form.HiddenField;
import org.apache.wicket.model.Model;
import org.apache.wicket.validation.IValidatable;
import org.apache.wicket.validation.IValidator;
import org.apache.wicket.validation.ValidationError;

import java.util.UUID;

/**
 * SyncTokenField
 */
@Log4j2
@StatelessComponent
public class SyncTokenField extends HiddenField {

    private static final long serialVersionUID = 5172245669993637549L;

    /**
     * Construct.
     *
     * @param id
     */
    public SyncTokenField(String id) {
        super(id);
    }

    @Override
    protected void onInitialize() {
        super.onInitialize();
        setType(String.class);
        setRequired(true);
        regenerateToken();
        add(new IValidator<String>() {
            @Override
            public void validate(IValidatable<String> validatable) {
                String token = (String) getModel().getObject();
                log.info("validatable.getValue() = {} || token = {}", validatable.getValue(), token);
                if (token == null || !token.equals(validatable.getValue())) {
                    ValidationError validation = new ValidationError(getString("messageMailDoubleSendError"));
                    error(validation);
                }
            }
        });
    }

    protected final void onComponentTag(final ComponentTag tag) {
        super.onComponentTag(tag);
        String token = (String) getModel().getObject();
        tag.put("value", token);
    }

    public String regenerateToken() {
        String token = UUID.randomUUID().toString();
        setModel(Model.of(token));
        return (String) getModel().getObject();
    }
}

regenerateToken メソッドはvalidation処理後のsubmit処理時、に実装する想定です。