Ok so lets take the two handed fighter archetype and it's ability Over hand Chop.
"At 3rd level, when a two-handed fighter makes a single attack (with the attack action or a charge) with a two-handed weapon, he adds double his Strength bonus on damage rolls."
Great, have a +5 from strength it becomes +10 awesome, fine. Now enters at level 6 Horn of the Criosphinx.
"Benefit(s): Whenever you make a successful charge attack while wielding a two-handed weapon in both hands, add two times your Strength bonus to the damage roll."
Does this mean that if I'm using Overhand chop and I charge that I get +20 damage on my attack?
This is the same as all of the spells/feats/abilities that double crit range. There are multiple ways to achieve the same effect, and adding more methods doesn't make the ability stronger.
It just means there is a way to get a magic item that does what your class ability does. There are tons of examples of this. The major benefit is that class abilities can usually be accessed earlier than the equivalent magic item. The trade off is you can't ditch a class ability when the item becomes available.