feat: disktable and memtable behave inconsistently when putting duplicate data

In memtable, for example, putting 3 same data will result in 3 records. In disktable, it will result in only 1 record.
Putting duplicate data in memtable is like creating and in disktable updating.

Look at the memtable and disktable implementations to understand their put logic for duplicate data. The issue mentions inconsistency: memtable inserts duplicates as separate records while disktable updates existing ones. Start by examining the code paths for put operations in both components, then decide on a unified behavior. Running existing tests related to data insertion will help verify the fix.

索引モデルが issue の本文から書いたものです。