Double old-path record will cause the import to throw a 500

[jjanssen]

In an occasion where an Excel list is handed over there is a possibility where a duplicate from-record exists. For example, having a file with over 1.000 records could have the following:

From	To
/my/previous/location/1	/my/new/path
/my/previous/location/1	/my/new/some-other-path

When this is present the importer will throw an error as it tries to save another record in which old_path is classified unique:

duplicate key value violates unique constraint "wagtailredirects_redirect_old_path_site_id_783622d7_uniq"
DETAIL:  Key (old_path, site_id)=(/my/previous/location/1, 3) already exists.
mechanism

Proposed solution
We should wrap this in a try-catch, process the file accordingly and notify we have skipped a record due to a duplication.