When there is a "flat" symbol on "re" despite already having a universal "flat" symbol on it ?

sorry my piano lingo isn't there as I'm self-taught, but there's a flat symbol on "re/d" already, but there's another one, and when i checked out a piano tutorial online, the guy just played it with one flat.. i dunno, i thought with 2 overlapping flats, it might like... uh... do something weird.