diff --git a/app/models/group.rb b/app/models/group.rb
index 6c0a5258..4b50b836 100644
--- a/app/models/group.rb
+++ b/app/models/group.rb
@@ -208,9 +208,18 @@ def default_user(uid)
     user = User.find_by(uid:)
     return user if user.present?
 
-    user = User.new(uid:, default_group_id: id)
-    user.save!
-    user
+    begin
+      user = User.new(uid:, default_group_id: id)
+      user.save!
+      user
+    rescue ActiveRecord::RecordNotUnique => unique_error
+      # If adding a submitter to an existing group
+      Rails.logger.error("Failed to created a new user for #{self}: #{unique_error}")
+      user = User.new(uid:)
+      user.default_group_id = id
+      user.save!
+      user
+    end
   end
 end
 # rubocop:enable Metrics/ClassLength
diff --git a/spec/models/group_spec.rb b/spec/models/group_spec.rb
index 5554e18a..007b641d 100644
--- a/spec/models/group_spec.rb
+++ b/spec/models/group_spec.rb
@@ -143,5 +143,23 @@
       Group.plasma_laboratory.default_user("abc123")
       expect(user.reload.default_group).to eq(Group.research_data)
     end
+
+    context "when an error is encountered while persisting the user model" do
+      let(:user) { FactoryBot.build(:user) }
+      let(:uid) { "abc234" }
+      let(:default_group_id) { Group.plasma_laboratory.id }
+
+      before do
+        user
+        allow(User).to receive(:new).with(uid:, default_group_id:).and_raise(ActiveRecord::RecordNotUnique)
+        allow(User).to receive(:new).with(uid:).and_return(user)
+      end
+
+      it "attempts to create a new user mode without initially setting the default group ID" do
+        persisted = Group.plasma_laboratory.default_user("abc234")
+
+        expect(persisted.default_group_id).to eq(Group.plasma_laboratory.id)
+      end
+    end
   end
 end