Swp files cause the current koan to be repeated

[James EdJo]

If there is a .01_ticket.rs.swp file then this gets added to the list of koans as a second instance of the current koan. This means that when tests pass it asks if you'd like to move onto the next koan but repeats the current one instead.

error[E0428]: the name `greetings` is defined multiple times
 --> jira-wip/src/path_to_enlightenment.rs:6:1
  |
3 | mod greetings;
  | -------------- previous definition of the module `greetings` here
...
6 | mod greetings;
  | ^^^^^^^^^^^^^^ `greetings` redefined here
  |
  = note: `greetings` must be defined only once in the type namespace of this module

For more information about this error, try `rustc --explain E0428`.
error: could not compile `jira-wip` (bin "jira-wip" test) due to previous error