Meaning that the table holds more than a single record with the requested Id.
True, this is the main reason for such errors. Many resolution are available for this errors.
1) Use Top 1 keyword i.e.
(SELECTTOP1 account_number FROM patients WHERE file_no = param_file_no);
(SELECTTOP1 account_number FROM accounts WHERE account_id = param_account_id);
(SELECTTOP1 price_list_id FROM accounts WHERE account_number = param_account_number);
2) Some of your data require 1 or more condition (filter) in your sql query.