Logstash imap plugin read a lot of Duplication email

[shawn_sun (sunyaxiong)]

version： logstash7.8.0 and elasticsearch7.8.0
When I use Logstash imap input plugin to read email from IMAP server，The elasticsearch recive many repeat email

this is logstash.conf

 input {
	imap {
	 	host => "imap.mailhost.com"
	 	user => "user@mailhost.com"
	 	password => "****"
	 	content_type => "text/plain"
	 	secure => true
	 	port => 993
	 	check_interval => 10
	 	folder => "Inbox"
	}
}
 
output {
	stdout { codec => rubydebug }
 
 	elasticsearch {
 		index => "emails"
 		document_type => "_doc"
 		hosts => "172.31.27.212:9200"
		user => "elastic"
		password => "****"
	}
}

When I perform "./bin/logstash -f logstash.conf “ ，meanwhile send a test email, I Can see many repeat email print in the console.

Please help me location the problem, Thanks.