JSON to Java Deserialization with same field name

I have tried the following snippet:

public class MainClass {

    public static void main(String[] args) {
        String json = "{n"
                + "  "_id": "a392ec9c-4f5c-411a-8599-928c5ce6b18d",n"
                + "  "name": "polName",n"
                + "  "rules": [n"
                + "    {n"
                + "      "t": "TestRule",n"
                + "      "principals": [n"
                + "        {n"
                + "          "name": "Test Report",n"
                + "          "id": "Test Report",n"
                + "          "type": "PY"n"
                + "        },n"
                + "        {n"
                + "          "name": "Test Report1",n"
                + "          "id": "Test Report1",n"
                + "          "type": "SY"n"
                + "        }n"
                + "      ]n"
                + "    }n"
                + "  ]n"
                + "}";
        try {
            PolicyDto p = new ObjectMapper().readValue(json, PolicyDto.class);
            System.out.println("P "+p.getRules().get(0).getPrincipals().get(0).toString());
        } catch (JsonProcessingException p) {
            p.printStackTrace();
        }
    }
}

And the DTO:

import com.fasterxml.jackson.annotation.JsonAlias;
import com.fasterxml.jackson.annotation.JsonIgnoreProperties;
import com.fasterxml.jackson.annotation.JsonInclude;
import com.fasterxml.jackson.annotation.JsonInclude.Include;
import com.fasterxml.jackson.annotation.JsonProperty;
import java.util.List;

/**
 *
 * @author asgar
 */
@JsonInclude(Include.NON_EMPTY)
@JsonIgnoreProperties(ignoreUnknown = true)
public class PolicyDto {

    @JsonProperty("docId")
    @JsonAlias({"_id"})
    private String id;

    private String name;

    private List<Rule> rules;

    public String getId() {
        return id;
    }

    public void setId(final String id) {
        this.id = id;
    }

    public String getName() {
        return name;
    }

    public void setName(final String name) {
        this.name = name;
    }

    public List<Rule> getRules() {
        return rules;
    }

    public void setRules(final List<Rule> rules) {
        this.rules = rules;
    }

    public static class Rule {

        private String t;

        private List<Principal> principals;

        public List<Principal> getPrincipals() {
            return principals;
        }

        public void setPrincipals(final List<Principal> principals) {
            this.principals = principals;
        }

        public String getT() {
            return t;
        }

        public void setT(String t) {
            this.t = t;
        }

    }

    public static class Principal {

        private String name;
        private String id;
        private String type;

        public String getName() {
            return name;
        }

        public void setName(final String name) {
            this.name = name;
        }

        public String getId() {
            return id;
        }

        public void setId(final String id) {
            this.id = id;
        }

        public String getType() {
            return type;
        }

        public void setType(final String type) {
            this.type = type;
        }

        @Override
        public String toString() {
            return "Principal{" + "name=" + name + ", id=" + id + ", type=" + type + '}';
        }

    }
}

The output:

P Principal{name=Test Report, id=Test Report, type=PY}

The code works fine and the id you mentioned above seems to be populated properly. How are you trying to use this DTO? Can you elaborate more?