I think this problem might be partly due to the documentation at
http://svn4j.org/javadoc/org/tmatesoft/svn/core/wc/SVNWCClient.html#doAdd(java.io.File,%20boolean,%20boolean,%20boolean,%20org.tmatesoft.svn.core.SVNDepth,%20boolean,%20boolean)
Notice that a quick reading of the documentation for doAdd has this line:
mkdir - does not throw exceptions on already-versioned items
A more careful reading, of course, reveals that the mkdir documentation has just been misplaced:
force - if true
mkdir - does not throw exceptions on already-versioned items
and the mkdir explanation above is actually for the force parameter.
It is quite confusing, however, and I had the same error as the original poster before finding this post via google and then reading the documentation a little more carefully to recognize the error.
I would submit that this documentation needs to be fixed.
