Short answer: Duplication is for smaller quantities and it’s faster. Replication is for when you want more and it’s still pretty fast.
More words: Duplication is the copying of data onto recordable media (CD-R, DVD-R etc). This term is generally used to mean small-run projects where data is “burned” onto a pre-made recordable disc. Like in every department, Furnace only uses Grade A materials. Master quality disc media = industrial quality drives and equipment- nothing but the best for you. Duplication pros: speedy (same-day-delivery-speedy), small runs (25-copies-small).
Replication is the manufacturing process of an optical disc (CD, DVD, Blu-Ray). This term is generally used to mean larger-scale press runs where your content in transferred to glass-master, plates are made and then used to press/mold polycarbonate into discs. Complicated stuff. Replication Pros: lower per piece cost (cheaper by the dozen and all that) ,, fancy offset or silk-screen disc face printing, and a larger variety of print and packaging options.