Yes, "had used" is a correct grammatical construction, and it isn't necessarily redundant. The term "had" is what's known as the past perfect tense. We use the past perfect when we...
What is the purpose and flexibility of the past perfect tense?
Of course! The past perfect tense is used to show that an action took place before another action in the past. It's structured with 'had' + Past Participle. For example: "I had eat...