Why? Dilute the Ilford fixer for the paper strength (9+1). Especially with RC paper, you don't need the extra strength and it would theoretically require more washing.
The rationale for using stronger fix is that you can fix the paper for a shorter period of time, which makes it easier to wash out than if it's immersed longer in weaker fix. I'm not sure whether this is needed with RC papers, though.
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