It sounds like the "Format" for the Default display is wrong. What is the "format" value shown inhttp://YOUR.DOMAIN/admin/structure/types/manage/YOUR_CONTENT_TYPE/display? Typically it should be "Default". If the Default Display is set to "Summary or Trimmed", then you'll get the effect you mention, with the Summary in both the Teaser and Default versions.
You can check this by changing the settings and doing Preview on the node edit page. Preview shows the Teaser and Default versions simultaneously.
Also, is "Custom Display Settings" checked for the Full Content, or just the Teaser? Typically it would be just the Teaser. If you have set Custom Display Settings for Full Content, then check the settings as above.
It sounds like the "Format" for the Default display is wrong. What is the "format" value shown inhttp://YOUR.DOMAIN/admin/structure/types/manage/YOUR_CONTENT_TYPE/display? Typically it should be "Default". If the Default Display is set to "Summary or Trimmed", then you'll get the effect you mention, with the Summary in both the Teaser and Default versions. You can check this by changing the settings and doing Preview on the node edit page. Preview shows the Teaser and Default versions simultaneously.
By the way, <!-- break --> works fine in Drupal 7. I use it.