Multiple mail received with live email notifications

Reproduction steps, on a fresh wiki with Jetty/hsqldb:

• Set grace time to 0 in xwiki.properties
• Configure the wiki to send email (from email address + email server) and set 0 to the time between emails
• Create a user with an email address
• Create a page "Page"
• Login with the user previously created
• enable all page notifications by alert and email
• enable mentions notifications by email (alert is already enabled by default)
• set email frequency to live
• Go to the page "Page"
• Unwatch the whole wiki with the watch buttons, and watch the current space
• Login again with Admin and create a new page "Sub page" under "Page" hierarchy

Expected result:

• a single mail is immediately received for the new page creation

Obtained result:

• I immediately obtained 3 emails for same event

Note that performing further editions also lead to multiple emails.

The issue was originally reported on https://forum.xwiki.org/t/emails-are-sent-twice-or-three-times/13073 as impacting 14.10.15 but I reproduced it on 14.10.16.