It seems that the editingSessionId field is regularly duplicated in wikitext edit sessions, but not VE sessions.
```
> SELECT
-> event_editor,
-> COUNT(*) AS sessions,
-> SUM(revisions > 1) AS cross_revision,
-> SUM(pages > 1) AS cross_page,
-> SUM(users > 1) AS cross_user
-> FROM (
-> SELECT
-> event_editingSessionId,
-> event_editor,
-> COUNT(DISTINCT `event_page.revid`) AS revisions,
-> COUNT(DISTINCT `event_page.id`) AS pages,
-> COUNT(DISTINCT `event_user.id`) AS users
-> FROM Edit_11448630
-> WHERE
-> timestamp BETWEEN "20150401" AND "20150402" AND
-> event_action = "saveSuccess"
-> GROUP BY 1
-> ) AS session_counts
-> GROUP BY 1;
+--------------+----------+----------------+------------+------------+
| event_editor | sessions | cross_revision | cross_page | cross_user |
+--------------+----------+----------------+------------+------------+
| visualeditor | 13416 | 0 | 0 | 0 |
| wikitext | 309795 | 627 | 3 | 0 |
+--------------+----------+----------------+------------+------------+
2 rows in set (49.02 sec)
```
I did some more digging. See my notes here: https://meta.wikimedia.org/wiki/Research_talk:VisualEditor%27s_effect_on_newly_registered_editors/Work_log/2015-04-13
TL;DR: 100% of dupe sessions have only one "init" event.