To convince someone to do something that they do anyways (idiom)

Preaching to the choir means trying to persuade an already convinced audience. The idiom is based on the idea of a pastor of a church delivering his sermon to the people in the choir, who are presumably already believers. Generally it's used to mean "you don't have to convince me," as in "Hey buddy, you're preaching to the choir here, I'm already voting Democratic this year" (or whatever).

(idiomatic) Speaking as if to convince a person or group of something which that person or group already believes. Often used to imply that a speaker is addressing the wrong audience or is deliberately addressing a compliant, non-challenging audience.

https://en.wiktionary.org/wiki/preach_to_the_choir

Preach to the converted:

• to try to persuade people to believe things they already believe (usually in continuous tenses).

  • There's no need to tell us about the benefits of recycling. You're preaching to the converted.

The Free Dictionary

The idiom "pushing at an open door" has that meaning:

Definition of push at or against an open door in English:

Have no difficulty in accomplishing a task: ‘if the management were to tackle this issue, it might find that it was pushing at an open door’

www.oxforddictionaries.com

For example, "trying to persuade that alcoholic to have a drink was pushing at an open door"...