is there any bonus to attatching two of the same materia(example two fire materia) together?

It depends - just having two spell materia together in the same slot won't have any effect, but it they're linked with support materia (such as Quadra-Magic), you can use them for different effects (for instance, say you have one Fire materia with Magic Counter materia in one linked slot, and another Fire materia linked with Quadra Magic in another slot, and finally a third Fire materia linked with Elemental in your armour. The end result would you'll be able to cast Fire/Fire2/Fire3 four times in the one turn, while also counterattacking with Fire 3 automatically when hit, and also be immune to fire elemental attacks. The key is using linked slots and support/command materia effectively.

An interesting addendum to this is the Enemy Skill command materia - it's useful to connect all four available E Skill materia to the same person, so that all four materia will learn that spell if that person is hit with the spell (this is especially helpful in having all four available E Skill materia learn rare spells like Trine, Shadow Flare and Pandora's Box). Just be aware that if one materia already has the spell you're trying to learn, then the other materia will not learn it even if the enemy uses it on you.

Just be aware that if one materia already has the spell you're trying to learn, then the other materia will not learn it even if the enemy uses it on you.

I never actually knew that.