Version: 49.0.2623.112
OS: Mac 10.11.4

What steps will reproduce the problem?
(1) Open https://vaadin.github.io/expense-manager-demo/
(2) Observe that https://vaadin.github.io/expense-manager-demo/elements/elements.html gets loaded from the network twice.

What is the expected output?

That https://vaadin.github.io/expense-manager-demo/elements/elements.html only gets loaded once.

What do you see instead?

That it gets loaded twice.

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