Past perfect tense and simple past tense.

By courtesy of GPT.
The difference between the two sentences lies in the tense and aspect of the verb “had.” Let’s break down each sentence:

“Last night I had had beer.”

This sentence uses the past perfect tense (“had had”). The past perfect tense is formed by using the past tense of the auxiliary verb “have” (had) followed by the past participle of the main verb (“had” in this case). It is used to indicate an action that occurred before another past action. In this sentence, it suggests that before a certain point in the past (last night), you had already consumed beer.
“Last night I had beer.”

This sentence uses the past simple tense (“had”). The past simple tense is used to describe completed actions that happened at a specific point in the past. In this sentence, it simply states that last night, you consumed beer.
So, the difference is that the first sentence emphasizes that the beer consumption occurred before some other past event, while the second sentence simply states that the beer consumption happened last night without reference to any other past event.