eliminate duplicate placeholders in "walkComparison" function within Que...

...ryExpressionVisitor

I've run into an issue when using Criteria with QueryBuilder when Criteria uses multiple checks on same variable in different context, example:
field1 = value OR (field1 in values AND field2 = true)... what happens is field 1 placeholder is duplicated, and an error occures in code complaining about invalid # of parameters.  The fix eliminates this issue, making sure parametername is unique.

I have not had time to check any other possible issues this may cause, but a review of the code doesn't appear to impact any other function.