Names referees served main referee at least two

List the names of referees who have served as the main referee in at least two matches.

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Database schema

CREATE TABLE "public"."teams" (
    "team_id" varchar(10) NOT NULL,
    "name" varchar(80) NOT NULL,
    "city" varchar(80) NOT NULL,
    "playground" varchar(80) NOT NULL,
    "jersey_home_color" varchar(80),
    "jersey_away_color" varchar(80),
    PRIMARY KEY ("team_id")
);

CREATE TABLE "public"."managers" (
    "mgr_id" varchar(10) NOT NULL,
    "name" varchar(80) NOT NULL,
    "dob" date NOT NULL,
    "team_id" varchar(10),
    "since" date,
    PRIMARY KEY ("mgr_id")
);

CREATE TABLE "public"."matches" (
    "match_num" varchar(10) NOT NULL,
    "match_date" date NOT NULL,
    "host_team_id" varchar(10) NOT NULL,
    "guest_team_id" varchar(10) NOT NULL,
    "host_team_score" int4 NOT NULL,
    "guest_team_score" int4 NOT NULL,
    PRIMARY KEY ("match_num")
);

CREATE TABLE "public"."match_referees" (
    "match_num" varchar(10) NOT NULL,
    "referee" varchar(15),
    "assistant_referee_1" varchar(15),
    "assistant_referee_2" varchar(15),
    "fourth_referee" varchar(15),
    PRIMARY KEY ("match_num")
);

CREATE TABLE "public"."referees" (
    "referee_id" varchar(10) NOT NULL,
    "name" varchar(80) NOT NULL,
    "dob" date NOT NULL,
    PRIMARY KEY ("referee_id")
);

CREATE TABLE "public"."players" (
    "player_id" varchar(10) NOT NULL,
    "name" varchar(80) NOT NULL,
    "dob" date NOT NULL,
    "jersey_no" int4 NOT NULL,
    "team_id" varchar(10) NOT NULL,
    PRIMARY KEY ("player_id")
);