Newbie question about db normalization theory: redundant keys OK?

With a few hours of theory under my belt, I'd like to ask if there's
ever a time that you don't want a completely normalized dB, that is, a
normalized database being a dB that has no redundant information (my
understanding of what a normalized database is).

Or, is there ever a time that you want redundant keys (that is, the
same keys in many different tables, that obviously are not linked (in
a relationship) between two tables?). Having redundant attributes and/
or keys seems to me a very lazy way of designing a database that
doesn't require lots of initial thought, but of course you have to pay
for it by meticulously "synching" all redundant keys to one another
everytime there is a change in one of the redundant keys, so the keys
don't drift and have different values.

But is there ever a time you want to do this?

THanks in advance

RL
.