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_SecondaryPositionNV and gl_PositionPerViewNV are inconsistently
declared inside and outside of gl_PerVertex. This breaks interface block
matching. For now ignore these errors since it should be fixed with how
they are declared.
