Add the possibility to use a named placeholder multiple times in a query

Currently, in a QSqlQuery, a named placeholder cannot be used multiple times.

An query like this is invalid:
QSqlQuery query;
query.prepare("SELECT x, y FROM z WHERE :price > 10 AND :price < 1000");
query.bindValue(":price", someVariable);

It would be useful to be able to use the same placeholder every time the value is needed.