Albert Barnes Commentary 1 Samuel 17:34

"And David said unto Saul, Thy servant was keeping his father`s sheep; and when there came a lion, or a bear, and took a lamb out of the flock," — 1 Samuel 17:34 (ASV)

The narrative does not make it certain whether the lion and the bear came on the same occasion or on two different occasions. If it was on one occasion, the probability would be that the bear, having seized a lamb and carrying it off, a lion appeared to dispute the prize with the bear, or with David after he had taken it from the bear, and that David killed first one and then the other.