Cover the case when pipeline has multiple scans of same type

ee/app/models/ee/merge_request.rb

@@ -260,7 +260,7 @@ module EE
     def missing_security_scan_types
       return [] unless actual_head_pipeline && base_pipeline
 
-      base_pipeline.security_scans.pluck(:scan_type) - actual_head_pipeline.security_scans.pluck(:scan_type)
+      (base_pipeline.security_scans.pluck(:scan_type) - actual_head_pipeline.security_scans.pluck(:scan_type)).uniq
     end
 
     private

ee/spec/models/merge_request_spec.rb

@@ -975,6 +975,15 @@ RSpec.describe MergeRequest do
               end
 
               it { is_expected.to eq(['sast']) }
+
+              context 'when there are multiple scans for the same type for base pipeline' do
+                before do
+                  build = create(:ci_build, :success, pipeline: base_pipeline, project: project)
+                  create(:security_scan, build: build, scan_type: 'sast')
+                end
+
+                it { is_expected.to eq(['sast']) }
+              end
             end
           end
         end