How does rune combiner work?
When combining 5 runes to get a higher-tier one, can you get runes that you already have unlocked 9 of, or are you guaranteed to get a rune that you are still missing?
As already covered in this question the runes you get are random.
And since they are random you can get one you already own a full set(9) of.
As already said, its random so you can get anything. You should probably try to combo first all lower tier runes, before buying all the runes you want.