Normal folk will never hit the limits, even if the hoard books

I think I have heard some folk have 60,000 entries
OTOH you need a decent PC and HD. An old 386 based desktop is not going to cut it.

Any 64bit system with Win 7 should be fine if it as 4G RAM and a decent HD
Duplicates? You only get those if YOU add them more than once,
Preferences: Adding Books has controls that might help (look at all the Tabs).
Find Duplicates Plugin can help FIND (you need to decide what to do with each)