HTML formatted emails have content repeated

OK so I was struggling for a while to get Simplenews to send HTML emails. I can now send emails in HTML format but the content of the emails sent by Simplenews is duplicated.

Note that to get Simplenews to send HTML emails I needed to manually edit database table simplenews_category (column 'format') from 'plain' to 'html' (I only have one row/category in this table). I am not sure if this is related to the problem I am experiencing.

The problem occurs regardless of whether I use Mimemail or HTMLmail (the 2 modules Simplenews suggests to use in order to send HTML formatted mail). These modules come with a test facility (sending html email but not through Simplenews) which work fine (send HTML emails with no duplicated content).

I have set up a new text format to use without any filters but this doesn't help either.

I have tried uninstalling and starting from scratch multiple times but to no avail...

Please, anyone, what might be going wrong!