$ cat script.sh 
#!/bin/bash
duplicate_user_list="rod jane freddy"
duplicate_user_count=3

if [ "$duplicate_user_count" -gt "0" ]
then
        echo "Duplicate users found, mailing you the details"
        echo "$duplicate_user_list" | mail -s "Details of duplicate users" unixforum@localhost
        exit 1
else
        echo "No duplicate users found"
        exit 0
fi

$ ./script.sh 
Duplicate users found, mailing you the details
$ mail
"/var/mail/unixforum": 1 message 1 new
>N   1 unixforum@localhost Thu Mar  9 13:30  15/667   Details of duplicate users
? 1
<Some headers redacted for privacy reasons>
Subject: Details of duplicate users
To: <unixforum@localhost>
X-Mailer: mail (GNU Mailutils 2.99.99)

rod jane freddy
? d 1
? q
Held 0 messages in /var/mail/unixforum
$