Raise error for redundant namespaces

[gazayas]

Closes #89.

I assume that we want to do this for all types of Action Models, but if there's anything extra I should do here I'd be glad to look into it.

[jagthedrummer]

@gazayas, should we add a test or two here to make sure that we continue to do the right thing around this issue?

[gazayas]

Joint PR

• https://github.com/bullet-train-co/bullet_train/pull/1102

[jagthedrummer]

@gazayas @andrewculver, just FYI, I just pushed version 1.2.12 with this fix to gem fury.