You could receive a message for existing tag if you already have a tag with the same name. For example, you have the following tags - "sport" and "music". If you try to rename "music" as "sport", you will receive "Tag 'sport' already exists.
Regarding the second issue, we removed target deliberately, as it is not XHTML compliant and we claim that Sitefinity provides valid XHTML code. Also, openenig a new browser window (target="_blank") is a usability issue for users of your site and it is in Top Ten Web Design Mistakes
according to usability experts as Jacob Nielsen. So, removing target, we are trying to keep sites developed by Sitefinity, standard compliant and more usable. However, if you need it, you can set target in the HTML view in the editor.
the Telerik team