Hi Olaf,
I believe this is a problem with the jBPM runtime because it uses the BPMN element "id" attribute instead of "name" for the variable name (presumably because XML Schema ID naming rules are similar to Java's variable names.) This was, IMHO, an unfortunate design decision which essentially means that ALL variable names must be unique within the process. I'm not sure if there's a bugzilla enhancement request filed for this or not, but the workaround is to rename your variables so they don't collide.
Bob