actually JRR Tolkien published only LOTR and the Hobbits as a
beginning of the story but he was planning the world middlearth
since he was at university. after he died his son took all the
writings and published them. Silmarillion and the other books
contains the whole partitions that JRR decided to put in the story.
but the other parts and ideas that not decided to put in the story
still exists. the unfinished tales contains this stories and ideas.
if you read this tales you can understand that they are
'unfinished' and not overlap the whole story.