Hello everyone!
I saw that a lot of people have my same issue.

So:
I wear a Valkyrie Helm (front and top accessories allowed) and I already got the goggles (front helm) on me.
I cannot equip Spike Mohawk (top helm) because he tells me that I already have a similar accessory equipped on me.
But they're from two differents slot so...
BUG. :(