If loop not working properly - bash

Question

In my script i execute correctly when first time in the if loop execute after the third time of execution its not working properly .what can i do is any mistake ah...? please fix my issue...?in the script i gave the value for customer name as ABC then again i gave the ABC that time it ececute the if condition correctly,but i gave the third time as ABC its not working properly..!
#!/bin/bash
echo " --- Enter the Database name ---"
read databasename
echo " --- enter the table name --- "
read table_name
sqlite3 $databasename.db "DROP TABLE IF EXISTS $table_name;"
sqlite3 $databasename.db "CREATE TABLE IF NOT EXISTS $table_name(cus_id INTEGER PRIMARY KEY AUTOINCREMENT UNIQUE,cus_name TEXT NOT NULL UNIQUE ,cus_domain TEXT UNIQUE, cus_status TEXT NOT NULL,Port INTEGER NOT NULL);"
echo " --- Enter the total number of rows do you want ---"
read cus_count
echo "--- Enter the following details one by one---"
port_num=8080
for((i=1;i<=cus_count;i++))
do
echo "enter the $i customer details"
echo "---Enter the customer name---"
read c_name
customer=$(sqlite3 $databasename.db "select cus_name from $table_name")
if [[ "$port_num" == "$port_num" ]]; then
port_num=$(($port_num + 1))
fi
if [[ $c_name != $customer ]]
then
echo "---Enter the Status(Active/Inactive)---"
read c_status
c_domain="$c_name"
else
echo "---OOPS you entered customer name already available---"
echo "---Please enter new customer name---"
i=$(($i - 1))
fi
sqlite3 $databasename.db "INSERT OR IGNORE INTO $table_name (cus_name,cus_domain,cus_status, Port) VALUES(\"$c_name\",\"${c_domain,,}.com\",\"$c_status\",\"$port_num\") ;"
done
echo " --- Records from the $table_name ---"
sqlite3 $databasename.db "select * from $table_name;"

Answer 1

please refer to below on "bash script arithmetic operations"
http://tldp.org/LDP/abs/html/arithexp.html
or
http://tldp.org/LDP/abs/html/ops.html