You have multiple (or no) directories from svn marked as place for branches.
Look into you .git/config file, there would be section like that:
[svn-remote "svn"]
url = file:///someurlto/svn
fetch = trunk:refs/remotes/trunk
branches = branches/*:refs/remotes/*
branches = branches2/*:refs/remotes/*
There will be multiple entries for branches. (Alternately, if you don't have any branch entries, the first line should work with a standard SVN repo layout.)
So, when branching you must point in which directory the branch should be created:
git svn branch someFeature -m "test" --destination branches2
where the last element is one of the directories from branches lines in .git/config.
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