Steps to reproduce:

1. Start the external import wizard for a CSV import
2. Upload a file and create a new configuration
3. Map some of the fields in the file to existing, distinct custom fields (make sure you have more than one custom field defined in your system)
4. Advance through the wizard until you are able to download the configuration file
5. Download the configuration file
6. Cancel the wizard and restart
7. Upload the configuration file and the same import file again

Upon reaching the field mapping screen, you will notice that for the fields which were mapped to existing custom fields, they are all mapped to the first custom field in the list.

See the screenshot for an example of what shows up in the UI, compared to this snippet of the configuration file that was generated:

field.signatory = customfield_10021
field.PSC_file = customfield_10003
field.corr_date = customfield_10004
field.corr_ref = customfield_10020
field.id = customfield_10022
field.ministry_desc = customfield_10000
field.assignee = assignee
field.cat_desc = customfield_10001
field.register_time = created
field.reporter = reporter
field.corr_desc = summary

Notice that the custom fields are distinct in the configuration file, but not in the UI.