WARNING: HORRIBLE EDITING AND RANDOM STEAM PING SOUNDS.
Some login issues caused this.
You can buy duplicates of everything (items, internals, weapons) but only the internals will show up, and they will be gone as soon you enter a match. So yeah, the duplicate items won't work and are just a waste of HC.
However, I belive this bug triggered the the weapon glitch, but this is just an hypothesis. It seems that the weapon glitch stopped working when I spawned twice with the same mech.
Thanks to hellc9943 and SparkyJJC for their help.