Create a
UNIQUE constraint on your database table for the relevant columns - name, address, phone number. This means the database will stop you creating duplicate records.
If you try to create a duplicate record, the SQL insert will fail and you will get an exception. You can catch this exception in your code and decide how to handle it.
Don't bother trying to check for duplicates just before you do the insert, because that is not reliable: somebody could insert a record with the same key values just after your check but just before your insert.
Only the database knows for sure if the record already exists, so let the database perform the check via the unique key constraint. That's exactly what it is designed for. It is also efficient, because the constraint is enforced via an index, so the check can be performed against the index instead of reading the whole table.