Currenty, when trying to use the same named query parameter in more than one place, Moodle reports an error such as:

ERROR: Incorrect number of query parameters. Expected 2, got 1.

This appear to happen because the code checking for the number of named parameters does not account for re-use:

$named_count = preg_match_all('/(?<!:[a-z][a-z0-9_]*/', $sql, $named_matches);

I have encountered this problem in Moodle 2.0, and the associated code appears to be unchanged in 2.1dev.