Note [Do not add duplicate quantified instances]

As an optimisation, we avoid adding duplicate quantified instances to the
inert set; we use a simple duplicate check using tcEqType for simplicity,
even though it doesn't account for superficial differences, e.g. it will count
the following two constraints as different (#22223):

  - forall a b. C a b
  - forall b a. C a b

The main logic that allows us to pick local instances, even in the presence of
duplicates, is explained in Note [Use only the best matching quantified constraint]
in GHC.Tc.Solver.Dict.