Duplicated Typeclass Instances

Recently I come across the following flag for Coq

(* When a typeclass resolution is launched we ensure that 
   it has a single solution or fail. This flag ensures 
   that the resolution is canonical, but can make proof 
   search much more expensive. *) 
Set Typeclasses Unique Solutions.

As far as I understand, it disables possibility to keep two instances of the same typeclass. I tried this flag in a random place and immediately found type-class duplication.

  Require Export prosa.analysis.facts.preemption.job.nonpreemptive.  (* Introduces non-preemptive preem. policy. *) 
  
  <...>

  Context '{JobPreemptable Job}.                                     (* Introduces an arbitrary preemption policy. *) 
  Hypothesis H_valid_preemption_model :                              (* Picks whatever it wants. *) 
    valid_preemption_model arr_seq sched.

With the proposed flag, Coq reports an error. Do we need this?