This question already has an answer here:
If someone already flagged a question as duplicate and there is already the auto-generated "possible duplicate" comment visible, like for example:
possible duplicate of How to compare Strings in Java
Is upvoting this duplicate-comment considered as a flag-as-duplicate or should I also flag the question?
For the case the upvote of the dupl-comment is not considered as a flag-as-duplicate: Maybe it should? It would be more convenient than clicking through the flagging menus. Also I think an upvote of the dupl-comment is a pretty clear expression already that the user agrees that it is a duplicate.
see also the related question here: Auto-generate the "possible duplicate" comment when question is flagged as a duplicate