It is invalid if the same decoration is applied to the same id multiple
times. This adds a check before adding a decoration that the decoration
is not already in the list. If it is, then the duplicate is not added.
Fixes#3627
Co-authored-by: Neil Hickey <neil.hickey@arm.com>
Co-authored-by: Stuart Brady <stuart.brady@arm.com>
Signed-off-by: Sven van Haastregt <sven.vanhaastregt@arm.com>