Count duplicate items from custom object - android

Question

I have a custom object list like bellow :

public List<Message> getMessages() {
    return messages;
}

And :

public class Message {

    @SerializedName("id")
    @Expose
    private String id;
    @SerializedName("useridfrom")
    @Expose
    private String useridfrom;
    @SerializedName("useridto")
    @Expose
    private String useridto;
    @SerializedName("subject")
    @Expose
    private String subject;
    @SerializedName("fullmessage")
    @Expose
    private String fullmessage;
    @SerializedName("fullmessageformat")
    @Expose
    private String fullmessageformat;
    @SerializedName("fullmessagehtml")
    @Expose
    private String fullmessagehtml;
    @SerializedName("smallmessage")
    @Expose
    private String smallmessage;
    @SerializedName("notification")
    @Expose
    private String notification;
    @SerializedName("contexturl")
    @Expose
    private Object contexturl;
    @SerializedName("contexturlname")
    @Expose
    private Object contexturlname;
    @SerializedName("timecreated")
    @Expose
    private String timecreated;
    @SerializedName("timeuserfromdeleted")
    @Expose
    private String timeuserfromdeleted;
    @SerializedName("timeusertodeleted")
    @Expose
    private String timeusertodeleted;
    @SerializedName("component")
    @Expose
    private String component;
    @SerializedName("eventtype")
    @Expose
    private String eventtype;
    @SerializedName("userfromfirstnamephonetic")
    @Expose
    private String userfromfirstnamephonetic;
    @SerializedName("userfromlastnamephonetic")
    @Expose
    private String userfromlastnamephonetic;
    @SerializedName("userfrommiddlename")
    @Expose
    private String userfrommiddlename;
    @SerializedName("userfromalternatename")
    @Expose
    private String userfromalternatename;
    @SerializedName("userfromfirstname")
    @Expose
    private String userfromfirstname;
    @SerializedName("userfromlastname")
    @Expose
    private String userfromlastname;

    public String getId() {
        return id;
    }

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

    public String getUseridfrom() {
        return useridfrom;
    }

    public void setUseridfrom(String useridfrom) {
        this.useridfrom = useridfrom;
    }

    public String getUseridto() {
        return useridto;
    }

    public void setUseridto(String useridto) {
        this.useridto = useridto;
    }

    public String getSubject() {
        return subject;
    }

    public void setSubject(String subject) {
        this.subject = subject;
    }

    public String getFullmessage() {
        return fullmessage;
    }

    public void setFullmessage(String fullmessage) {
        this.fullmessage = fullmessage;
    }

    public String getFullmessageformat() {
        return fullmessageformat;
    }

    public void setFullmessageformat(String fullmessageformat) {
        this.fullmessageformat = fullmessageformat;
    }

    public String getFullmessagehtml() {
        return fullmessagehtml;
    }

    public void setFullmessagehtml(String fullmessagehtml) {
        this.fullmessagehtml = fullmessagehtml;
    }

    public String getSmallmessage() {
        return smallmessage;
    }

    public void setSmallmessage(String smallmessage) {
        this.smallmessage = smallmessage;
    }

    public String getNotification() {
        return notification;
    }

    public void setNotification(String notification) {
        this.notification = notification;
    }

    public Object getContexturl() {
        return contexturl;
    }

    public void setContexturl(Object contexturl) {
        this.contexturl = contexturl;
    }

    public Object getContexturlname() {
        return contexturlname;
    }

    public void setContexturlname(Object contexturlname) {
        this.contexturlname = contexturlname;
    }

    public String getTimecreated() {
        return timecreated;
    }

    public void setTimecreated(String timecreated) {
        this.timecreated = timecreated;
    }

    public String getTimeuserfromdeleted() {
        return timeuserfromdeleted;
    }

    public void setTimeuserfromdeleted(String timeuserfromdeleted) {
        this.timeuserfromdeleted = timeuserfromdeleted;
    }

    public String getTimeusertodeleted() {
        return timeusertodeleted;
    }

    public void setTimeusertodeleted(String timeusertodeleted) {
        this.timeusertodeleted = timeusertodeleted;
    }

    public String getComponent() {
        return component;
    }

    public void setComponent(String component) {
        this.component = component;
    }

    public String getEventtype() {
        return eventtype;
    }

    public void setEventtype(String eventtype) {
        this.eventtype = eventtype;
    }

    public String getUserfromfirstnamephonetic() {
        return userfromfirstnamephonetic;
    }

    public void setUserfromfirstnamephonetic(String userfromfirstnamephonetic) {
        this.userfromfirstnamephonetic = userfromfirstnamephonetic;
    }

    public String getUserfromlastnamephonetic() {
        return userfromlastnamephonetic;
    }

    public void setUserfromlastnamephonetic(String userfromlastnamephonetic) {
        this.userfromlastnamephonetic = userfromlastnamephonetic;
    }

    public String getUserfrommiddlename() {
        return userfrommiddlename;
    }

    public void setUserfrommiddlename(String userfrommiddlename) {
        this.userfrommiddlename = userfrommiddlename;
    }

    public String getUserfromalternatename() {
        return userfromalternatename;
    }

    public void setUserfromalternatename(String userfromalternatename) {
        this.userfromalternatename = userfromalternatename;
    }

    public String getUserfromfirstname() {
        return userfromfirstname;
    }

    public void setUserfromfirstname(String userfromfirstname) {
        this.userfromfirstname = userfromfirstname;
    }

    public String getUserfromlastname() {
        return userfromlastname;
    }

    public void setUserfromlastname(String userfromlastname) {
        this.userfromlastname = userfromlastname;
    }

}

How can I count duplicate items : useridfrom and useridto.

I like use from RXJava for better performance an do in background.