Using the same parameter name in a thread can cause values to be lost

In the following structure, the use of the same parameter name ("-> knot") causes an issue. When the choice inside "level two" is selected, and the flow loops back into level_one.opt, the value of the first "knot" parameter is overwritten with value from the level two knot.

If the parameters are given different names, then the bug doesn't occur.

```
- (top)
-> level_one (-> end )
- (end)
-> END

=== level_one (-> knot)
- (opt)
<- level_two(-> opt)
* [ Knot variable is {knot} ] -> knot

=== level_two(-> knot)
* [ choosing this causes the "knot" variable to change value ]
-> knot
```