Detect duplicated keys on CompoundTag deserialization

[dktapps]

While no sane implementation should allow this, there is a possibility in the CompoundTag binary format for the same key to appear multiple times. If this occurs, the last tag will overwrite the previous ones, or potentially crash due to mismatched type.

This should be detected as data corruption and exceptions thrown appropriately instead.

[dktapps]

Fixed by 1ba981d.