Debconf, package.config script and subst+multiselect

[Yurkao <yurkao@gmail.com>]

Dear mentors, 
As I've understood from debconf(7):
 - package config script runs twice during the deb installation
As a result:
 - package config script must be idempotent and must not/should not ask questions twice.

As I understand,I should control not asking question twice by checking if return value of db_input equals to 30.

If so, what if I should "render" multiselect choices of template with db_subst?
How do I avoid calling db_subst on second config run ? Should I avoid calling db_subst on second run?

What is the best way to avoid asking questions twice? By checking result of db_input=30 or by calling 'db_fget question seen'?

Best regards,
Yurii Oleynikov