For example, I withdrew 5 MIOTA from Bitfinex to my light wallet seed. I see 6 duplicate transactions, 1 of which went through and is reflected in my total balance, but the other 5 saying pending. What are these pending transactions, why do they exist, and is it okay that they are there?
These are Reattaches of the same transaction. Only one of them can confirm, the others will stay pending forever. They'll eventually get pruned away during the next snapshot.