If the same queue event already exists in the file monitor queue discard it.

This fixes an out of memory error if a watched file is changed
multiple times in a second. In this case the queue gets filled faster as
it can be processed.
10 jobs for master in 14 minutes and 11 seconds (queued for 2 seconds)
latest
Status Job ID Name Coverage
  Build
passed #161455
cross-android_api21_arm64

00:02:30

passed #161456
cross-android_api28_arm64

00:02:30

passed #161457
cross-mingw64

00:02:55

passed #161454
fedora-autotools-x86_64

00:06:07

passed #161453
fedora-x86_64

00:06:36

passed #161458
win32
msys2-mingw32

00:11:13

passed #161459
win32
vs2017-x64

00:07:25

 
  Coverage
passed #161460
coverage

00:02:04

75.3%
 
  Deploy
passed #161461
pages

00:00:53

passed pages:deploy

00:00:01