What is "mechanistic duality"?
This just means that the same type of reaction may proceed by two different mechanisms depending on the exact chemical being used. It's not a particularly widely used term (it is not a "standard" phrase such as wave-particle duality) and there are many other ways of saying the same thing ("divergence", "pathway", etc.).

As an extremely simple example, alkyl bromides can react with nucleophiles via either an SN1 or SN2 pathway, depending on the degree of substitution of the carbon bonded to bromine. This is an example of how a different substrate may follow a different mechanism, although the end result is the same (replacement of Br with the nucleophile).

The papers you cited are just slightly more sophisticated examples of the same concept.