AsTreeCTE复用报错

asTreeCTE表达式复用时，比如`cloneQueryable`和`union`，会导致创建两次with语句从而报错

```
var sql = asTreeCTE();
sql.union(sql).toList()
```
只`union`可能不报错但是`toList`会报错

Start by locating the implementation of asTreeCTE and trace how cloneQueryable, union, and toList build and render CTEs. Reproduce the issue with the provided snippet, then compare the generated SQL or query state when the CTE expression is reused. Done means reuse no longer creates duplicate WITH clauses and both union and toList execute successfully.

Written by the indexing model from the issue text.