I just reached level 20 to where I can buy tier 3 runes. (haven't bought one yet) but I was wondering if I could have the same rune on the same page?

You can have the same rune on the same page, but have to buy how ever many of those runes you want on that page (e.g. if you wanted all seal of potency you would have to buy 9 seals of potency(or w.e the ap runes are called))

I think at 20 you can have 6 red marks, 6 yellow seals, 6 blue glyphs and two Quints.

You can have 6 of the same marks, seals or glyphs and two of the same Quints.

I believe marks are primarily attack, seals defence and glyphs magic.

At level 30 you get 3 Quints and 9 of each of the others.