This stuff has been going on for decades between Sentai & KR.
I also remember the pretty "deep" sewer system in KR 555's final eps and in Hibiki for the sting ray makamo.
Dude...they can totally get away with re-hashing locations.
Each KR and Sentai series are stand-alone series, with the current one (Gekiranger in this case) being completely unrelated to the previous (Boukenger, for example).
...but I'm sure everyone knows this already. ^^;;