investigate duplicate hashes

[ctb]

in spacegraphcats/spacegraphcats#375, I discovered that bbhash seems to not detect duplicate input hash values and so any duplicates end up being ignored in terms of value setting.

investigate and confirm this behavior, and then test and document it!

[ctb]

also in relation to spacegraphcats/spacegraphcats#375, check what happens if we try to assign a value to a hash that doesn't exist in the mphf.