Bug 29346: Highlight double enqueue

Bug 29346: Highlight double enqueue

This patch highlights a possible issue with the triggers.. though it may
not actually matter in reality.

I appear to already see test failures before this patch with a double
enqueue of the rebuild for both AddIssue and AddReturn.. I couldn't spot
what was causing that but whilst digging I did find another case where
it could happen as highlighted here...

Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>