build fails when not doing "build clean"

[Joerg Heinicke]

Tried a "build webapp" (without clean). The build fails in target 
"patch-conf":

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Processing: D:\cocoon-2.1\src\blocks\session-fw\conf\session.xconf
Writing: D:\cocoon-2.1\build\webapp\WEB-INF\cocoon.xconf
Processing: 
D:\cocoon-2.1\src\blocks\authentication-fw\conf\authentication.xconf

Error in: D:\cocoon-2.1\src\blocks\authentication-fw\conf\provider.xconf

BUILD FAILED
file:D:/cocoon-2.1/build/cocoon-2.1m3-dev/temp/blocks-build.xml:5588: 
IOException: java.io.IOException: XPath 
(/cocoon/session-context-providers) returned not one node, but 2 nodes

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Exectuting "build webapp" once more and the count goes to 3 nodes and so on.

Joerg