The Kingston Trio predated the Chad Mitchell Trio by a couple of years; in fact, Chad Mitchell and Dave Guard apparently crossed paths at Stanford in around 1956 when Chad was looking to form a group and Guard had already done so.   I think the CMT may have started out copying the Kingstons, but once Milt Okun began his tenure as their musical director they sought to distinguish themselves from the pack and went more in the direction of comedy and political satire.

    Most of the "folk scare" trios had songs like "Reuben James," "Greenland Whalers," and "Whistling Gypsy" in their set lists. My sense is that they probably did these songs because they were good songs that worked for them, and perhaps because audiences expected to hear songs such as these. In the case of the CMT, "Gypsy" worked because is allowed Chad to show off his whistling prowess, while the other songs were good since they let each member of the group take a verse while they all harmonized on the choruses. Also, as you mentioned, both trios had their own sound and would never have been mistaken for one another even if they'd had the same song out as a single at the same time (I don't know if that ever happened).

    As for copying the Weavers or the Almanacs, you could probably say that all the folk revival groups were doing that to some extent, if for no other reason than that's how the folk process works.

                                       'Ray