SearchArgumentImpl generates wrong ExpressionTree in some cases which might result in loss of data

ExpressionBuilder applies `pushDownNot`, `foldMaybe`, `flatten`, `convertToCNF`, `flatten` and `buildLeafList` in order to form a non-normalized expression into a CNF expression with the unique leaves.

After an expression is converted to CNF, there might be more than one non-leaf nodes which are exactly the same object in the expression tree. If this happens, those non-leaf nodes will be visited more than once in `buildLeafList` function. As a result, a wrong ExpressionTree is generated.

My version is 1.2.1, but it seems that the higher versions are also affected