Deduplicate in_network_ids before VF lookup

in_network_ids is passed to the VF client without deduplication, while
oon_ids is deduped four lines below. retweeted_tweet_id is pushed for
every candidate that has one, so the same ID repeats once per retweet of
a given post — most often when that post is going viral.

Neither VfClient implementation dedupes its input: StratoVfClient builds
one call per element, and XaiVfClient chunks by XAI_VF_MAX_BATCH_SIZE, so
duplicates consume batch slots and can force an extra round trip.

Not a correctness issue — results collapse into a HashMap keyed by tweet
ID — but redundant work on the For You serving path.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>