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
gl_SubGroupARB was being correctly declared as an Input variable in
Vulkan but it was missing the Flat decoration, which made spirv-val emit
the VUID-StandaloneSpirv-Flat-04744 validation error with shaders using
that built-in.