When tagging a matrix project with two slaves on an axis, each build tries to tag the source code.
The first build successfully deletes the existing tag and creates a new tag.
The second build fails to delete the existing tag and creates a trunk folder inside the existing tag.

Console output of the first build:

Started by upstream project "test" build number 6
Building remotely on slave1
Checking out a fresh workspace because there's no workspace at /tmp/jenkins_slave1/workspace/test/label/slave1
Cleaning workspace /tmp/jenkins_slave1/workspace/test/label/slave1
Checking out svn+ssh://user@server/repo/test/trunk
AU abc.txt
At revision 238
no change for svn+ssh://user@server/repo/test/trunk since the previous build
moduleLocation: Remote ->svn+ssh://user@server/repo/test/trunk
Tag Base URL: 'svn+ssh://user@server/repo/test/tags/myTag'.
Delete old tag svn+ssh://user@server/repo/test/tags/myTag.
Tagged as Revision 240
Notifying upstream projects of job completion
Finished: SUCCESS

Console output of the second build:

Started by upstream project "test" build number 6
Building remotely on slave2
Checking out a fresh workspace because there's no workspace at /tmp/jenkins_slave2/workspace/test/label/slave2
Cleaning workspace /tmp/jenkins_slave2/workspace/test/label/slave2
Checking out svn+ssh://user@server/repo/test/trunk
AU abc.txt
At revision 238
no change for svn+ssh://user@server/repo/test/trunk since the previous build
moduleLocation: Remote ->svn+ssh://user@server/repo/test/trunk
Tag Base URL: 'svn+ssh://user@server/repo/test/tags/myTag'.
There was no old tag at svn+ssh://user@server/repo/test/tags/myTag.
Tagged as Revision 241
Notifying upstream projects of job completion
Finished: SUCCESS

Subversion log says:

239 Deleted /test/tags/myTag "Old tag deleted by Jenkins build: jenkins-label=slave1
240 Added /test/tags/myTag (Copy from path /test/trunk) "Tagged by Jenkins build: jenkins-label=slave1
241 Added /test/tags/myTag/trunk (Copy from path /test/trunk) "Tagged by Jenkins build: jenkins-labe-=slave2