Why HTTP method PUT should be idempotent and not the POST in implementation RestFul service?

There are many resources available over the internet wherein PUT vs POST is discussed. But I could not understand how would that affect the Java implementation or back end implementation which is done underneath for a RestFul service? Links I viewed are mentioned below:

https://www.keycdn.com/support/put-vs-post/

https://spring.io/understanding/REST#post

https://www.w3.org/Protocols/rfc2616/rfc2616-sec9.html

http://javarevisited.blogspot.com/2016/10/difference-between-put-and-post-in-restful-web-service.html

For example let's say there is a RestFul webservice for Address.So POST /addresses will do the job of updating the Address and PUT /addresses/1 will do the job of creating the one.Now how the HTTP method PUT and POST can control what weservice code is doing behind the scenes?

PUT /addresses/1 

may end up creating multiple entries of the same address in the DB.

So my question is, why the idempotent behavior is linked to the HTTP method?

How will you control the idempotent behavior by using specif HTTP methods? Or is it that just a guideline or standard practice suggested?

I am not looking for an explanation of what is idempotent behavior but what make us tag these HTTP methods so?