Calendar Portlet duplicates itself while paging through months

[cdw9]

Happens in Plone 5.0.7

• Add a calendar portlet to the root of the site
• View the portlet in the site
• hit the button for next month a couple times
• after a couple months, a second calendar portlet will appear

At this point, if you hit 'next' again, the entire page will reload showing the next month and one calendar portlet. If you hit 'next' on the new portlet, you will soon get 4 calendar portlets.

[mauritsvanrees]

Confirmed in current Plone coredev 5.0.
It does not happen on 5.1, nor on 4.3 (when using plone.portlets 2.2.3 due to a different issue).

No idea why this happens.