The most common reason is that you are trying to log in or sign in using Safari. With Safari, you will be asked to accept cookies before logging or signing in. Sometimes, users are asked several times in a row to accept these cookies.
You can disable the "Prevent Cross-Site tracking" button in your settings ( > Safari > Privacy & Security). After that, you will no longer be asked by third parties to accept cookies multiple times.
If you don't want to disable "Prevent Cross-Site tracking", try logging in using another browser such as Mozilla Firefox, Internet Explorer, Google Chrome, or change your device, e.g., laptop, tablet, or cell phone.