Apple Pay failed: Page already has an active payment session.

Currently, I'm developing a web service that provides Apple Pay functionality, so I'm trying Apple Pay Demo on this page.

https://applepaydemo.apple.com/

And I found that Apple Pay failed on a certain device with the following error message.

> InvalidAccessError: Page already has an active payment session

I'm using:

- MacOS 10.14.6 (18G84)

- Safari 12.1.2 (14607.3.9)

I searched for some information but couldn't find anything useful. I also checked WebKit source code, but still not sure what's wrong.

https://github.com/WebKit/webkit/blob/master/Source/WebCore/Modules/applepay/ApplePaySession.cpp#L523-L524

Does anyone have an idea?