Suppose there are two users that have commented on an answer. Both have the same user name, and I want to reply to one of them. How can I do this?

It appears that, under the current system, there is no way for me to do this. Specifically, from How do comment @replies work?, I see that:

4. Matching is performed in reverse chronological order, so if five people named John are participating, @john will match the most recent John

So, consider this a request to add this feature using an -n suffix or something similar; e.g. @john-2 could be used to reply to the second John from the top.

Update
Thanks to @Lix for nice answer of @name(userid) based auto-complete suggestion which has no loophole. It involves no counting from top & has no consequences when a comment is deleted. But, I think @name(userid) should be used directly in comment as all form inputs at SE are text (nothing embedded).

Now, I'm trying to finalize it based on it:
The system should allow optional @name(userid) format for reply. Provided userid with this format, system should ignore name. And, in the situation of duplicate name based confusion, system should demand this format as mandatory to submit comment.

How to get userid?
In case of duplicate name based confusion, system should append (userid) in hypertext to their profile. It'd work for mobile version too.
On full version, for easy fill-up, a hover reply icon would be fine (like hover vote up icon). Upon clicking it, system should fill @name(userid) in comment text box.

From reply reader side:
Multiple Johns could be confused even if they didn't come from notification (because nobody remembers his/her own userid). So, highlight the background of comments which are replied to them.

Suppose there are two users that have commented on an answer. Both have the same user name, and I want to reply to one of them. How can I do this?

It appears that, under the current system, there is no way for me to do this. Specifically, from How do comment @replies work?, I see that:

4. Matching is performed in reverse chronological order, so if five people named John are participating, @john will match the most recent John

So, consider this a request to add this feature using an -n suffix or something similar; e.g. @john-2 could be used to reply to the second John from the top.

Update
Thanks to @Lix for nice answer of @name(userid) based auto-complete suggestion which has no loophole. It involves no counting from top & has no consequences when a comment is deleted. But, I think @name(userid) should be used directly in comment as all form inputs at SE are text (nothing embedded).

Now, I'm trying to finalize it based on it:
The system should allow optional @name(userid) format for reply. Provided userid with this format, system should ignore name. And, in the situation of duplicate name based confusion, system should demand this format as mandatory to submit comment.

How to get userid?
In case of duplicate name based confusion, system should append (userid) in hypertext to their profile. It'd work for mobile version too.
On full version, for easy fill-up, a hover reply icon would be fine (like hover vote up icon). Upon clicking it, system should fill @name(userid) in comment text box.

From reply reader side:
Multiple Johns could be confused even if they didn't come from notification (because nobody remembers his/her own userid). So, highlight the background of comments which are replied to them.