Probably one of them might be closed as a duplicate of the others. (The question is which choose as the duplicate target - a natural choice might be the older one, OTOH I'd guess that my answer is a bit more detailed than the answers to the older post. robjohn's answer also contains some outdated information - but that is mentioned in the comments.)
And I also thought about adding newcommand to the older post - in order to make it easier to find in the future. (Possibly one of the posts - or both of them - could be also added into the tag-wiki.)
Maybe closing as a duplicate + merging could also be an option. (The older question has a title which makes it more likely to be found if somebody searches for this information.)
And maybe also we could say that the two posts are different enough not to be considered duplicates. (If we read one of them as "How can I do this?" and the other one as "I know that I can use \newcommand, but I do get blank lines - what to do?")
