Only return PrometheusAlert if singular found

lib/gitlab/alerting/alert.rb

@@ -122,10 +122,13 @@ module Gitlab
       def parse_gitlab_alert_from_payload
         return unless metric_id || gitlab_prometheus_alert_id
 
-        Projects::Prometheus::AlertsFinder
+        alerts_found = Projects::Prometheus::AlertsFinder
           .new(project: project, metric: metric_id, id: gitlab_prometheus_alert_id)
           .execute
-          .first
+
+        return if alerts_found.empty? || alerts_found.size > 1
+
+        alerts_found.first
       end
 
       def parse_title_from_payload

spec/lib/gitlab/alerting/alert_spec.rb

@@ -79,11 +79,26 @@ describe Gitlab::Alerting::Alert do
         )
       end
 
-      before do
-        payload['labels'] = { 'gitlab_prometheus_alert_id' => second_gitlab_alert.id }
+      context 'alert id given in params' do
+        before do
+          payload['labels'] = { 'gitlab_prometheus_alert_id' => second_gitlab_alert.id }
+        end
+
+        it { is_expected.to eq(second_gitlab_alert) }
       end
 
-      it { is_expected.to eq(second_gitlab_alert) }
+      context 'metric id given in params' do
+        # This tests the case when two alerts are found, as metric id
+        # is not unique.
+
+        # Note the metric id was incorrectly named as 'gitlab_alert_id'
+        # in PrometheusAlert#to_param.
+        before do
+          payload['labels'] = { 'gitlab_alert_id' => gitlab_alert.prometheus_metric_id }
+        end
+
+        it { is_expected.to be_nil }
+      end
     end
   end