Merging annotations gets messy when there are duplicate values or conflicts in keys

Duplicate values in annotations tends to result in GumTree mapping the wrong values together (for some reason). As keys are not represented at all in the GumTree tree, this typically results in one of the key/value pairs just disappearing. I don't intend to fix this as far as the thesis goes.

Start by reproducing a merge involving duplicate annotation values and conflicting keys, then inspect how GumTree maps the annotation values when keys are absent from its tree. Done means both key/value pairs remain correctly represented after merging; the issue notes that this was not intended to be fixed for the thesis.

Written by the indexing model from the issue text.