Add validate: false. Add validates uniqueness.

app/models/ci/pipeline_schedule.rb

@@ -9,7 +9,7 @@ module Ci
     belongs_to :owner, class_name: 'User'
     has_one :last_pipeline, -> { order(id: :desc) }, class_name: 'Ci::Pipeline'
     has_many :pipelines
-    has_many :variables, class_name: 'Ci::PipelineScheduleVariable'
+    has_many :variables, class_name: 'Ci::PipelineScheduleVariable', validate: false
 
     validates :cron, unless: :importing?, cron: true, presence: { unless: :importing? }
     validates :cron_timezone, cron_timezone: true, presence: { unless: :importing? }

app/models/ci/pipeline_schedule_variable.rb

@@ -4,5 +4,7 @@ module Ci
     include HasVariable
 
     belongs_to :pipeline_schedule
+
+    validates :key, uniqueness: { scope: :pipeline_schedule_id }
   end
 end