Web data contains millions of near-identical documents, news syndications, privacy disclaimers, and boilerplate footers. Deduplication prevents memorization, reduces loss spikes, and saves thousands of GPU hours.
Document-Level MinHash + LSH
Documents are split into n-gram sets (shingles). MinHash computes signature vectors. Locality-Sensitive Hashing (LSH) hashes signature bands into buckets to catch near-duplicates with Jaccard similarity > 0.8 without pairwise comparisons.
Line-Level & Sub-String Dedup
Common headers, footers, and cookie banners span across otherwise distinct documents. Suffix arrays locate repeated byte spans >= 50 tokens across multi-terabyte datasets.
Impact on Pre-Training
Deduplication reduces corpus size by 20%-50% while improving downstream benchmark accuracy, as models spend FLOPs learning concepts rather than repeating memorized text.