File systems prevent duplicate uploads primarily through deduplication techniques. Deduplication identifies and eliminates redundant copies of the same data, storing only a single instance while tracking multiple references to it. This differs from simply checking file names or paths; instead, it typically involves calculating unique identifiers (like cryptographic hashes) of the file content. If the hash matches an existing file, the upload is identified as a duplicate, and only a pointer or reference to the existing data is stored.

This technology is extensively used in cloud storage platforms like Dropbox or iCloud to efficiently store user files, especially when multiple users upload identical popular files. Similarly, enterprise backup systems rely heavily on deduplication to minimize the storage footprint required for repeated backups of large datasets, dramatically reducing backup times and storage costs.

The primary advantage is significant storage space savings and reduced network traffic during uploads. Limitations include the computational overhead of calculating hashes, especially for large files, and the potential risk of data loss if the single stored instance becomes corrupted. Future developments focus on improving efficiency with variable-length chunking and real-time deduplication during file transfer.