What is the meaning of нийлсэнсэн?
I saw the following sentence.
Энэ үед л бүх хүн түүнтэй санал нийлсэнсэн.

I guess нийлсэнсэн is related with нийлэх.
What is the meaning of нийлсэнсэн?
Why double сэн is used?

Question

What is the meaning of нийлсэнсэн?
I saw the following sentence. 
Энэ үед л бүх хүн түүнтэй санал нийлсэнсэн.

I guess нийлсэнсэн is related with нийлэх.
What is the meaning of нийлсэнсэн?
Why double сэн is used?

Answer

санал нийлэх - to agree
бүх хүн санал нийлсэн - everyone agreed
бүх хүн санал нийлсэнсэн - I remember everyone agreed (this is the rough meaning. -сэнсэн can be used to describe an event remembered from a nostalgic memory)