Is there, game mechanically speaking, a limitation of benefits from mods outside of hard point, equipment and common sense requirements? I get you can't get the benefit of multiple barrel attachments on a single barrel weapon, but if I had the hard points available could I attach 2 Utility Arm mods on the same suit of armor and gain the benefits from them both?

Generally speaking, you can only apply each type of attachment once.

I don't think it's explicitly stated anywhere in the rules, but instead is a general assumption that you can't stack attachments of the same type to get the same benefit multiple times.

Plus, it's generally just cheesy bending of the intent of the rules to do such a thing in the first place.

Although I would allow multiple gunner droid brains on a ship, one for each weapon system.

I think this is one of those ones where whether or not it's reasonable is fairly obvious. Multiple copies of an attachment stacking bonuses is unreasonable (e.g. multiple Utility Arms granting multiple Boost dice), multiple copies of an attachment each granting different bonuses is reasonable (e.g. multiple Weapon Mount attachments with different weapons).