The first solution that came to me is to use upsert to update ElasticSearch:

Use the primary-key as ES document id
Upsert the records to ES as soon as you receive them. As you are using upsert, the 2nd record of the same primary-key will not overwrite the 1st one, but will be merged with it.

Cons: The load on ES will be higher, due to upsert.

To use Flink:

Create a KeyedDataStream by the primary-key
In the ProcessFunction, save the first record in a State. At the same time, create a Timer for 15 minutes in the future
When the 2nd record comes, read the 1st record from the State, merge those two, and send out the result, and clear the State and the Timer if it has not fired
When the Timer fires, read the 1st record from the State and send out as the output record.
Have a 2nd Timer of 6 hours (or more) if you are not using Windowing to clean up the State

Pro: if you have already having Flink ingesting this stream. Otherwise, I would just go with the 1st solution.

My Stack