Current behavior:
When marking a payslip batch as paid, a ValueError "Expected singleton" is raised if a salary attachment and a manually added input line use the same code (e.g., OTHERFEES).

Expected behavior:
Batch should be marked as paid successfully even when multiple salary lines and multiple input lines share the same code.

Required Modules
- l10n_ch_hr_payroll_elm
- l10n_ch (Switzerland - Accounting)
- l10n_ch_hr_payroll (Switzerland - Payroll)
- l10n_ch_hr_payroll_account (Switzerland - Payroll with Accounting)
- l10n_ch_hr_payroll_elm (Switzerland - Payroll ELM Standard)
- l10n_ch_reports (Switzerland - Accounting Reports)

Steps to reproduce:

1. Activate the Payroll app
2. Install the required Swiss payroll modules listed above

3. Go to Payroll > Configuration > Salary Rules and create a new rule:
Name: Other Fees
Code: OTHERFEES
Category: Allowance
Salary Structure: Switzerland: Regular Pay
Active: Yes
Appears on Payslip: Yes
Condition Based on: Python Expression
Python Condition: result = 'OTHERFEES' in inputs
Amount Type: Python Code
Python Code: result = inputs['OTHERFEES'].amount and result_name = inputs['OTHERFEES'].name

4. Go to Payroll > Configuration > Salary Attachment Types and create a new type:
Name: Other Fees
Code: OTHERFEES
Country: Switzerland

5. Go to Payroll > Contracts > Salary Attachments and create an attachment:
Employee: (select an employee)
Type: Other Fees (the one you just created)
Description: e.g., "Parking"
Monthly Amount: e.g., $70.00
Start Date / End Date: set dates that cover your payslip period
State: Running

6. Go to Payroll > Payslips > All Payslips and create a new payslip:
Select the employee from step 5
Set the period to match your salary attachment dates
Add the payslip to a batch
Save and verify the salary attachment appears automatically in "Other Inputs"

7. In the "Other Inputs" section, add a second line:
Click "Add a line"
Create a new Other Input Type with code OTHERFEES (e.g., "Other Fees - Just Created")
Set a description (e.g., "Dining") and amount (e.g., -99)
You should now have TWO input lines both using code OTHERFEES

8. Click "Compute Sheet" and verify you see TWO salary lines with code OTHERFEES in the Salary Computation tab
Click "Create Draft Entry"
Go to Payroll > Payslips > Batches, find your batch, and click "Mark as Paid"
Expected ResultBatch is marked as paid successfully.Actual ResultValueError: Expected singleton: hr.payslip.line(??, ??)

Cause of the issue:
In _record_attachment_payment() (line 149), the code filters salary lines by code and expects a single record. When multiple lines share the same code, slip_lines contains multiple records, causing the singleton error on slip_lines.total access.

Fix:
Handle multiple salary lines with the same code by summing their totals/quantities instead of expecting a singleton.

opw-5473648