Using update_form on a custom form with duplicate fields

[spwoodcock]

Describe the bug

• We have a mandatory_fields.xlsx XLSForm.

• If this is merged via update_forms with a custom form that contains duplicate fields, then we get some duplicate columns.

• For example the mandatory_fields.xls contains the status field. The current buildings.xls also contains a status field.

• When these forms are merged, we get a duplicate save_to column called save_to.1:

  

To Reproduce

• Run update_form.merge_sheets on buildings.xls in this repo.
• Save the output to a file and view.

Expected behavior

• We should warn users they have a duplicate field when running the function.
• Ideally we return an error and ask them to rename / remove the field.
• In the end, the mandatory forms field should take precedence.

Additional context

• The form still works fine with the save_to.1 column.
• This isn't high priority as it doesn't break anything (as far as I am aware).