"Sahara Desert" secretly means "Desert Desert"

The word Sahara comes from the Arabic ṣaḥrāʾ, which simply means "desert." So every time you say Sahara Desert, you're technically saying "Desert Desert." It's not a mistake — it's a tautological place name, and once you notice them, they're everywhere.

England's River Avon is "River River" (afon is Welsh for river). Los Angeles' La Brea Tar Pits works out to "the tar tar pits." Lake Tahoe borrows the Washo word dá'aw — "lake" — so it's "Lake Lake." The Mississippi River doubles up the Ojibwe misi-ziibi, "big river."

These pile-ups happen through language layering. Newcomers hear a local name for a feature, don't realize the name already describes that feature, and tack on their own generic word — "river," "lake," "mountain." Centuries and several languages later, the redundancy is baked in and nobody blinks. Place names, it turns out, are fossils: each layer records who passed through and what language they spoke.