How to change 'termFreq' in BM25?

I use elasticsearch 5.6.
I am trying to reconstruct the Okapi BM25 default similarity in Elasticsearch.
Some of our document have repeat words in the database, such as "you are a good boy boy boy".
so, the 'termFreq' of the 'boy' word in BM25 is 3 when computing score. I want to set this to 1.
I found to use 'index_options=docs" could disable term frequency, but the "avgFieldLength" was changed to 1 too.
How can I only change 'termFreq'?
Thanks for any help.

can anybody answer my question?

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