What is the difference between "love one another" and "love your neighbour"?

How have “love one another” and “love your neighbour” been traditionally compared or harmonized? The former occurs in the Gospel and Epistles of John, which never say “love your neighbour”. The latter is in Matthew, Mark, Luke, and James, which do not refer to “love each other”. According to John 13:34, Christ said: > A new commandment I give unto you, That ye love one another; as I have loved you, that ye also love one another. If this is a new commandment, it must encompass something different from the old commandment in Leviticus 19:18: >Thou shalt not avenge, nor bear any grudge against the children of thy people, but thou shalt love thy neighbour as thyself: I am the LORD. In the early post-apostolic literature, only “love one another” appears in Second Clement, Minucius Felix, Aristides, and the Epistle of the Apostles, while the Didache, Justin Martyr, and Barnabas exclusively say “neighbour”. Both expressions are found in the Pauline Epistles, in Clement of Alexandria, and Tertullian. The two appear closely together, in adjoining sentences, only in Romans 13.8-9 and Clement’s Stromata 2.15. Complicating the issue were Polycarp, Irenaeus, and the Gospel of Thomas, who used neither expression, but say only “love your brother”. First John 2.10, 3.14, and 4.21 also say “love your brother” while other verses in the Epistle exhort “love one another”. With such a wide usage of both phrases, how have they been traditionally compared and contrasted? Do they find harmony or are they different phrases commanding different things?