In addition to what Marlana said about these words being the same as nouns, as actions or verbs they're a little different. I can backup my homework assignment, so I can spare myself of a headache later. Backup as a verb is like, copying something to keep it safe. Spare as a verb is like, saving something or someone. I can spare you from confusion by answering your question. I can also spare a cat crossing the street by stopping my car to not hit it.

They are the same. They both mean to have something that is usually a duplicate in case you need it. But we associate each word with something that it naturally seems to be known for. For example: we would say "a spare tire". We don't say "a backup tire". If you said "backup tire" as an English-learner, we would understand, but it wouldn't be accepted.

When used as nouns, backup means a reserve or substitute, whereas spare means the act of sparing

Big thanks you all🙏❤