Currently when checking for duplicate keys, we insert the value of the
key as is in a set then check for unicity against those. When the key is
an object, we check for duplicates based on object identity, whereas the
keys are used by owl as strings, and so using objects can cause
duplicate key errors that do not throw correctly but crash in the owl
internals.
This commit fixes that by making the duplicate checking code serialize
the key to string before insertion and when comparing against existing
keys.
Samuel Degueldre
d546244fc3