I got same problem with Kentico 10.
I'm using Editable Text in master page template. I've checked all webparts in Design tab, in Edit regions and webparts and in Webparts tab of the template, they're all same.
A notice is only webparts in 1 Webpart Zone have this issue. Other webparts (editable text) in other Webpart Zone are working just fine.
Everytime I save the page, the webpart adds up its content, separated with previous content by a comma.
I add an editable text to the zone with content: abc. After I click Save, the webpart content becomes abc,
I click Save again, the webpart content will become abc,abc
Appreciate if you can advise something.