This issue started after upgrading from Tiki 2.latest to 3.0.
Whenever an edit of a wiki page is saved the page contains the updated content, and then the old version after that. Basically it is as if the old content were not replaced by the new version but instead just as if the updated version were pasted in front of the old version.
This happens when doing "edit page" but not if doing an "edit section".
Any hint as to what might cause this or in which way I can continue to investigate will be greatly appreciated.