Why are duplicate activities being created?
Not sure if it's the same problem but it could be the submitOnce problem which has a temporary fix in 5.14.1 and a fuller fix pending but is an easy change you can make:

See https://github.com/civicrm/civicrm-core/pull/14545/files

i.e. remove the "submitOnce" line.

Briefly the popup version of activities conflicts with the extra javascript that submitOnce adds. The problem was reproduced on firefox - just create an activity using the popup version of e.g. a meeting, from the contact's record.