Excalibur and Caliburn do seem to be the same sword. It is a linguistic evolution. Both are taken to be Arthur's "main" sword, regardless of writer or name. The Sword in the Stone, and the one from the Lake, may or may not be the same sword. Clarent is not Excalibur, and is a different sword, wielded by King Arthur at points (and later, Mordred). More @Wikipedia
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