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The term "Duplicate Key" is used to describe an
index in a table that allows multiple records to
have the same key. The internal ordering of the identical keys in the index is
not guaranteed.
A consequence of this is that it is not possible to do a SeekBy and be guaranteed you
will end up with a specific record matching that key, this record may change over
time as the table is being modified.
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When a key is not a duplicate key, it is called a "Unique Key". They are
sometimes also called "Identity keys" because these types of keys can be used to
identify a specific record in that table.
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If a unique key is also a "null key", the uniqueness ONLY
holds for records that are in the index. Records that are not in the index are not required
to have key segment values that would make them unique if they would be in the index.
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