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Actually, the Hrehor theory is still likely. There are usually more derivatives of a name - such as, I'm Hana, called Hanka, but my aunt sometimes calls me Hanýžka - and there are also more variants of surnames derived from a name, such as Ondra and Ondøíèek, both derived from Ondøej (all examples from Czech). So, similarly, they could have easily called themselves differently to distinguish themselves from the already existing Hrehovs.
I'm not saying it was really like that, I'm just trying to explain how Slavic languages work...