Variables in CS-D may not always have unique names

The CS-D refactor introduced a nasty regression, uncovered by upgrade to the latest Kogito (a prerequisite to PLANNER-2131).

Turns out, old Kogito had no problems with two join variables having the same name, while the new one silently does. When such a thing happens, indexing breaks down and all sorts of weird rule firings are experienced as a result.