Fix #4121: No weapon switching when next and previous weapon are bound to the same key

[iManGaaX]

Summary

Binding the same key to both next weapon & previous weapon no longer switches the weapon

Motivation

Both controls were sent to the game in the same frame, so it cycled a weapon forwards and backwards at once and glitched the switch. they r now cancelled out: when both are down, neither is reported, so the current weapon stays

Closes #4121.

Test plan

Bind Q to both Next weapon and Previous weapon, then press Q in game; the weapon should not change; unbind Previous weapon: Q should cycle to the next weapon again

Checklist

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