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No worries. I think the Cybermen/Yeti section should go, if unsourced "rumours" aren't sufficient to support examples. Without a real explanation connecting the two, I don't think Monster X in Sewer can automatically be considered an expy of Monster Y in Subway. (Besides, as written, I don't think it's an example of an Expy anyway.
A more generic description of the first part, if it helps: An episode features supporting characters Alice and Bob. They are set to return for a later episode, but the actors aren't available at the time of filming, so the parts are (marginally) rewritten for two new characters, Alexis and Bert, who fill virtually the same roles as Alice and Bob were supposed to. This is all explicitly stated by Word of God in official material.
Personally, I don't think that makes them Expies; I'd call them Suspiciously Similar Substitutes.
All in all, this whole question is more about the distinction between an Expy and a Suspiciously Similar Substitute than the accuracy of sources.
EDIT: Oh hey, the page description for Expy explicitly states that a character can't be an Expy of a previous character from the same show: That's the exact definition of a Suspiciously Similar Substitute. Face Palm. Editing the example now.
Edited by SolipSchism

Can someone help me parse this example from Doctor Who S6 E3 "The Invasion"?
At the very least, if nothing gets removed, I'm going to separate this into two sub-bullets, since it's two separate examples.
Thanks in advance to any who reply.