diff --git a/app/models/spree_cm_commissioner/guest.rb b/app/models/spree_cm_commissioner/guest.rb
index 1c795a8af..29ac87870 100644
--- a/app/models/spree_cm_commissioner/guest.rb
+++ b/app/models/spree_cm_commissioner/guest.rb
@@ -180,23 +180,22 @@ def bib_display_prefix?
     end
 
     def generate_bib
-      return if bib_prefix.present?
-      return unless bib_required?
-      return if event_id.blank?
+      return if bib_prefix.present? || !bib_required? || event_id.blank?
 
-      self.bib_prefix = line_item.variant.bib_prefix
+      self.class.transaction do
+        rows = self.class.where(event_id: event_id, bib_prefix: line_item.variant.bib_prefix).lock
+        last_bib_number = rows.map(&:bib_number).max || 0
 
-      last_bib_number = event.guests
-                             .where(bib_prefix: bib_prefix)
-                             .maximum(:bib_number) || 0
+        self.bib_prefix = line_item.variant.bib_prefix
+        self.bib_number = last_bib_number + 1
+        self.bib_index = "#{event_id}-#{bib_prefix}-#{bib_number}"
 
-      self.bib_number = last_bib_number + 1
-      self.bib_index = "#{event_id}-#{bib_prefix}-#{bib_number}"
+        save!
+      end
     end
 
     def generate_bib!
       generate_bib
-      save!
     end
 
     # bib_number: 345, bib_prefix: 5KM, bib_zerofill: 5 => return 5KM00345
diff --git a/spec/models/spree_cm_commissioner/guest_spec.rb b/spec/models/spree_cm_commissioner/guest_spec.rb
index 89aa06990..9318aff49 100644
--- a/spec/models/spree_cm_commissioner/guest_spec.rb
+++ b/spec/models/spree_cm_commissioner/guest_spec.rb
@@ -309,7 +309,6 @@
       end
     end
 
-
     context "when guest in the same event" do
       context "when guest has the same prefix" do
         let(:line_item) { create(:line_item, order: order, variant: product_with_bib1.variants.first) }
@@ -327,6 +326,23 @@
         end
       end
 
+      context "when multiple generate_bib! concurrently" do
+        let(:line_item) { create(:line_item, order: order, variant: product_with_bib1.variants.first) }
+        let(:guest1) { create(:guest, line_item: line_item, seat_number: 1) }
+        let(:guest2) { create(:guest, line_item: line_item, seat_number: 2) }
+        let(:bib_prefix) { line_item.variant.bib_prefix }
+
+        it 'ensures unique bib_numbers and bib_index for each guest' do
+          threads = []
+          threads << Thread.new { guest1.generate_bib! }
+          threads << Thread.new { guest2.generate_bib! }
+          threads.each(&:join)
+
+          bib_indices = SpreeCmCommissioner::Guest.where(event_id: guest1.event_id, bib_prefix: bib_prefix).pluck(:bib_index)
+          expect(bib_indices.uniq.size).to eq(2)
+        end
+      end
+
       context "when guest has different prefix" do
         let(:line_item1) { create(:line_item, order: order, variant: product_with_bib1.variants.first) }
         let(:line_item2) { create(:line_item, order: order, variant: product_with_bib1.variants.last) }