diff --git a/lib/mail/notify/mailer.rb b/lib/mail/notify/mailer.rb
index 9deab1f..0cd46bd 100644
--- a/lib/mail/notify/mailer.rb
+++ b/lib/mail/notify/mailer.rb
@@ -76,9 +76,12 @@ def view_mail(template_id, options)
         subject = options[:subject]
         headers = options.except(:personalisation, :reply_to_id, :reference)
 
-        # we have to render the view for the message and grab the raw source, then we set that as the
+        # We have to render the view for the message and grab the raw source, then we set that as the
         # body in the personalisation for sending to the Notify API.
-        body = mail(headers).body.raw_source
+        #
+        # We do not pass the headers as the call to `mail` will keep adding headers resulting in
+        # duplication when we have to call it again later.
+        body = mail.body.raw_source
 
         # The 'view mail' works by sending a subject and body as personalisation options, these are
         # then used in the Notify template to provide content.
diff --git a/spec/mail/notify/mailer_spec.rb b/spec/mail/notify/mailer_spec.rb
index 622c89d..d72c68b 100644
--- a/spec/mail/notify/mailer_spec.rb
+++ b/spec/mail/notify/mailer_spec.rb
@@ -132,6 +132,20 @@
       expect(message.header[:reply_to_id]).to be_nil
       expect(message.header[:reference]).to be_nil
     end
+
+    it "sets custom headers only once" do
+      message_params = {
+        template_id: "template-id",
+        to: "test.name@email.co.uk",
+        subject: "Test subject",
+        custom_header: "custom header value"
+      }
+
+      message = TestMailer.with(message_params).test_view_mail
+
+      expect(message.header["custom-header"]).to be_a(Mail::Field)
+      expect(message.header["custom-header"].value).to eq("custom header value")
+    end
   end
 
   describe "#template_email" do
@@ -236,6 +250,20 @@
       expect(message.header[:reply_to_id]).to be_nil
       expect(message.header[:reference]).to be_nil
     end
+
+    it "sets custom headers only once" do
+      message_params = {
+        template_id: "template-id",
+        to: "test.name@email.co.uk",
+        subject: "Test subject",
+        custom_header: "custom header value"
+      }
+
+      message = TestMailer.with(message_params).test_view_mail
+
+      expect(message.header["custom-header"]).to be_a(Mail::Field)
+      expect(message.header["custom-header"].value).to eq("custom header value")
+    end
   end
 
   describe "#blank_allowed" do