関係代名詞の前の前置詞について

This is not a piece of advice to you I'm talking to myself for my practice of writing skills.
I don't know the source of this sentence, but I suppose next two sentences are both grammatically not wrong.
there might not be any problem for us to treat this sentence any way, but the writer must have had any intention/thought/feeling in writing this sentence. it may be this writer to be able to decide it reluctantly/unreluctantly.
He lost this award last year in which he won another one.(relative pronoun).
He lost this award last year which he won another one. (conjunction).
it'd be very good idea to dig out the old questions if we don't have any question.
Let's try this way more and more.

last year は　副詞句で　そこに関係代名詞をつなげる　という時点で　どういうことか　わからなくなります。last year は副詞句で　in をつけない　ところからのご質問でしょうが
いろいろなものが混在した　独自文法になってるような　

あえて言えば、which は　award のことか　と考え込んでしまいます。

This is not a piece of advice to you I'm talking to myself for my practice of writing skills.
At one time, schools taught students that a sentence should never end with a preposition.
there are times when following this particular grammar rule creates unclear or
Since the purpose of writing is to clearly communicate your ideas, it is acceptable to end a sentence with a preposition
Example: The car had not been paid for. (Ends with a preposition but is acceptable)
Unclear Revision: Paid for the car had not been. (Unclear sentence.)
However, you may decide that it is worth revising your sentences to avoid ending with a preposition
Example: My research will focus on the community the students lived in.
Revision: My research will focus on the community in which the students lived.
Example: I like the people I am working with.
Revision: I like the people with whom I am working.
:Preposition Basics.
https://academicguides.waldenu.edu/writingcenter/grammar/prepositions

必要です。英語を直します。

He did not win this award last year, during which he won another one.

しかし、関係代名詞でつなぐ必要はありません。これで十分です。

He did not win this award last year. He won another one.

This is not a piece of advice to you I'm talking to myself for my practice of writing skills.
I've searched about relative pronoun on the web but I couldn't find decisive explanation. so I write my idea. if theme sentence haven't preposition, I suppose this relative pronoun'd be equal to conjunction. without noticing/not knowing necessity of preposition I may use relative pronoun without preposition. I suppose person who'd notice the necessity of it should put the preposition. I suppose the sentence without preposition in spite of being necessary is wrong.