Taht's because it logs the enter and the exit, so, usually, when a process instance ends, you end up with to registers per node. The NodeInstanceLog object has a type attribute. You could use it to determine if you are "in" the node or you just "passed" the node. If you have ENTER but not EXIT, you are in.
nodeInstanceLog.getType() == NodeInstanceLog.NodeInstanceLog.TYPE_ENTER
nodeInstanceLog.getType() == NodeInstanceLog.NodeInstanceLog.TYPE_EXIT
Hope this help.